1. Mike Trout
Last year, I picked Mike Trout and I had a list of statistical reasons why I thought he might regress. There was evidence that he had gotten a tiny bit lucky on his line drives and that measures of outfield defense, which powered a good chunk of his 10-win rookie season, aren't really reliable from year to year. There was no doubt that Mike Trout was a very good player, but I figured he might lose a win or two (a catastrophe for most players) and become just another ho-hum eight win player/MVP candidate. Mr. Trout apparently didn't get the memo and put up another ten win season.

This year, my argument is more along the lines of "He can't really be a consistent 10-win player… can he?" I mean, at that point, we're talking about Willie Mays or Babe Ruth or Ted Williams. It seems like a bit of hubris to even hope that I'm in the middle of witnessing something that amazing. It feels like I'm cheating on the ghosts of the past, and in a sport obsessed with the past, that's always feels a little weird. So, maybe I'm just more hoping that Mike Trout becomes a seven- or eight-win player this year so that he's only a "really, really good player" rather than "an inner-circle Hall of Famer emerging from his cocoon before our very eyes." —Russell A. Carleton

2. Hunter Pence
This feels weird to say, because Hunter Pence built his career on consistency to the point at which you’d never project him for a breakout or a collapse. His first three seasons, his home run total ran: 25, 25, 25. After that, 22, 24, 27. His slashes weren’t that out of character last year either, .283/.339/.483, giving him an incredibly similar career line of .285/.339/.476.

But there he was at the end of last season with a career-high 5.6 WARP—a huge improvement from the 1.3 from the previous year. It wasn’t his defense, though that was some of it. It was the fact that in moving from the righty’s paradise of Houston to every hitter’s paradise of Philadelphia to San Francisco, he hasn’t paid the price for the ballpark yet. Maybe he won’t this year either. Maybe there was a breakthrough last year that allowed the career high in home runs despite the tough ballpark. (He hit 10 at home and 17 on the road.) But the defense will come down from the career-high 14.8 FRAA, and at some point, the unadjusted averages will start to tell a different story. —Zachary Levine

3. Shin-Soo Choo
Fresh off signing his seven-year, $130 million contract, the Scott Boras client is my pick as a regression candidate for 2014. Before Rangers fans chuck rocks into my window, just hear me out. Choo could very well turn in yet another 20/20 season, something he's done with regularity when healthy. But I just cannot see him defeating Grandfather Regression when it comes to his slash line.

Last year, Choo finished the regular season with a triple slash of .285/.423/.462—which is fantastic. If you showed me that line, I would say that player walks a whole lot. And Choo does; he walked a career high 15.7 percent of the time last year, upping his career average to 12.2 percent. What isn't so obvious, however, is the number of times Choo found himself getting plunked, and that's where the regression comes in.

From his career debut to prior to last season, Choo was plunked in 1.85 percent of plate appearances. The rate nearly doubled in 2013, jumping to 3.65 percent. While he certainly finds himself receiving a free base more often that most, Choo isn't in the Carlos Quentin division of receiving punishment from a baseball (3.98% career HBP rate). I'm convinced last season was just a fluke, and while it could happen again in 2014, I'm betting it doesn't. Regression! —Ronit Shah

4. A.J. Burnett
The revival of A.J. Burnett in Pittsburgh, coming off a barely-over-replacement-level season for the Yankees in 2011, was one of the better stories of the past couple of years. With the help of pitching coach Ray Searage, Burnett reinvented himself as a ground-ball pitcher; his 56.5 percent worm-killer rate was the second-highest among starting pitchers last year, topped only by Justin Masterson. But amid the transformation, Burnett retained his ability to miss bats; his 26.1 percent strikeout clip in 2013 marked a career high. And Burnett did all of this with his age clock ticking toward 40. He turned 37 on January 3.

But age isn't my concern here—and, truth be told, Burnett himself isn't, either. The trouble is that by signing with the Phillies, who topped the Orioles and Pirates for his services even though they are the least likely of the three clubs to contend, Burnett put himself in a situation in which his newfound ground-ball style is likely to become a liability. The Pirates allowed the fifth-lowest batting average on grounders last year (.224). The Phillies coughed up the second-highest (.260). And Philadelphia's infielders—most notably Chase Utley, the only defensive asset of the four—aren't getting any younger.

Opponents who hit the ball on the ground against Burnett last year wound up with a .240 average, right in line with the league mean. That number is almost certainly going to climb as he moves across the Keystone State. And unless Burnett reinvents himself again, his ERA is likely to go with it. —Daniel Rathman

5. Chris Davis
The longest nail always gets hammered.

Chris Davis is remarkable for this breakout in 2013, but I've learned not to expect aberrations to become a new norm. Am I to throw out all my prior knowledge on Davis?

Granted, I only have access to his results and not the process he underwent to get these results. So maybe I'm wrong. But I'm not a hater; I still think he'll be a valuable player. Now, about that Samardzjia guy… —PECOTA (via Harry Pavlidis)

6. Yasiel Puig
Yasiel Puig is an amazing athlete with a terrific future ahead of him in baseball. He is also a 23-year-old player who will be challenged even more by tougher pitchers this year. Puig’s super-fast start in June came before the pitchers adjusted to his game; his .272/.363/.453 slash from July 3 forward is likely a more realistic expectation than simply a repeat of 2013.

Another thing that stands out for Puig is the quality of pitchers he homered against. Last year Puig hit dingers against: Clayton Richard, Cory Gearrin, Paul Maholm, Adam Warren, Jason Marquis, Adam Ottavino, Juan Perez, Curtis Partch, Hector Rondon, Dan Jennings, Michael Bowden, Dale Thayer, and Tim Stauffer. While every team throws sub-par pitchers out there at some point during the season, Puig seemed to get more than his fair share.

Finally, Puig’s HR/FB percentage is extremely high for such a prolific ground-ball hitter (50.2 percent). He has enough strength to hit a number of home runs anyway, but a fall due to the lack of a fly-ball hitter’s profile wouldn’t be surprising in the least. —Mike Gianella

7. Jered Weaver
Weaver has been outpacing his peripherals for a while now, but he has a list of potential issues that threaten to undo the magic balancing act that’s made him as good as he is. His four-seam fastball averaged under 87 mph and will probably fall further in 2014. At some point, that is going to clash frightfully with his extreme fly-ball tendencies, especially as his slider has lost effectiveness in recent years. His 2013 strikeout rate was two percent lower than this career average and probably won’t get much better in the future. Sample sizes be damned, his LD% was also a career-high last year.

His awkward mechanics are also more likely to become an issue now that he’s on the other side of 30. A history of back and shoulder tightness worries me that an extended DL trip (aside from a freak accident like last year’s broken elbow) could be on the way.

No offense to PECOTA, but I’m thinking 3.6 WARP is optimistic. —Dan Rozenson

8. Jonathan Papelbon
Papelbon's sub-3.00 ERA in 2013 obscured a continued erosion in stuff. The Phillies' closer held on to his mid-90s heat through his final season with the Red Sox in 2011, but his velo took a tumble in 2012, and that decline only accelerated last year.

Mike Fast found that "the performance of a reliever appears to be more sensitive to changes in fastball speed," and sure enough, Papelbon's strikeout rate sank by 10 percentage points last season, to a career-low 22.4 percent. That's a problem for a guy who doesn't get groundballs. Papelbon's career 184 ERA+ compares favorably with Mariano Rivera's 180 through the same age, but if his drop in velo and rise in contact rate continue, ages 33 through 43 aren't going to be anywhere close to as kind. —Ben Lindbergh

Thank you for reading

This is a free article. If you enjoyed it, consider subscribing to Baseball Prospectus. Subscriptions support ongoing public baseball research and analysis in an increasingly proprietary environment.

Subscribe now
You need to be logged in to comment. Login or Subscribe
Fun article. We'll maybe not for these players....
Look at the rate of speed decline in Papplebon's slider -- down to curveball speeds. I wonder if he is using it differently, it's not really the same pitch, or if it is intentional.
He modified it to be more slurvy. Early on it was kind of like a cutter. He even called it a "slutter."
How about Mr. Boston, Pedroia?
Pedroia might qualify for a rebound list, since he played through a left thumb ligament tear almost all of last year.
Always missing from lists like this are the slightly bad guys who move to being terrible. Usually, you can find them on my Strat-O-Matic teams.
Travis Wood? Alex Cobb? Big regressions coming
Would love to hear about clues that Alex Cobb is due for regressing.
totally unsustainable LOB% of 81.4 for example
What about Cobbs MLB experience combined with a swing and miss change up that is one of the top 5 swing and miss pitches in the AL ??? No regression here for Alexander. He is just good.
He will still be a good pitcher, but a good pitcher with a 3.50ish ERA as opposed to a 2.74 ERA. That is regression, but still a very good ballplayer, nothing to be ashamed of there.
I thought Marlon Byrd would be on here for sure.
Shouldn't you have a reason other than "he can't possibly keep up the pace" when you project a regression for a guy like Trout? Seems a bit silly to me...
I have no specific reason to call for a Trout regression other than some sort of existential discomfort. There's always the element of Waiting for Godot in baseball. The past was always glorious and someday, another all-time great will emerge, but that time is not now. That's my entire reasoning.
why would someone not want to see Mike Trout become "an inner-circle Hall of Famer emerging from his cocoon before our very eyes." ?????
Something about hubris ...when in Rome ...
"A consistent 10 win player. . .like Willie Mays."

Mays had six 10 win seasons in 22 years, so I am not quite sure what your definition of "consistent" is. . . Ted Williams managed 5 ten WAR years in 19 seasons. . . So, assuming Trout lasts another 15 seasons, he'd only need 3 more 10 win seasons to be as "consistent" as Mays and Williams. . . .

And -- so what? If Trout "regresses" 30% to being a 7 win player in 2014, then 1) he will still probably be among the five most valuable players in baseball, and 2) 99%+ of the people who read this stuff for purposes of fantasy baseball would likely still draft him first or second knowing in advance that he'd be "only" a 7 win player.

And -- your "argument" is really just a question, a question that we won't know the answer to for many years.

I think the basis for your pick is really not about Trout per se so much as it is "to have the best chance to be 'correct' in identifying someone who is likely to regress, I might have the best chance if I select the person coming off of the best season in 2013."

LOL. Mays had six 10 WAR years between 1954 and 1965, in addition to two years of 9.9 WAR; the other years, his worst total was 8.3. Hard to call that inconsistent. A 10 WAR year is pretty unusual, so we'll see if he measures up to Mays. Not impossible, but certainly improbable, given that most would put Mays as the top CF in history.
I took WAR data for Mays from

Mays' 10 WAR seasons were 1954, 1958, and 1962-65.

The point isn't that Mays wasn't great, or consistently great, but that 10 WAR seasons are exceedingly rare - Mays achieved that level in 6 of 22 seasons, with those seasons occurring in the first half of a long career

Trout is playing at Mays' level at age 21 -

Sure, he'll regress, but regression from the level of performancechechas achieved so early in his career would seem to argue that he IS more likely than any other young player to be "inner circle" than not.

The "argument" that "he can't be that good, can he?" just seems weak to me, is not supported by any facts in the piece, and seems to be at odds with the facts we have to judge Trout to this point.
Okay, I used the BP WAR numbers; the 10+ seasons are the same, and they came in 12 seasons. There's also the fact that Mays's 21 and 22 year old seasons were lost to military service.

I guess my point is, trying to say that Trout's career so far is comparable to Mays's career is premature. Maybe he will have four more 10+ WAR seasons in the next seven years, but that's pretty much unprecedented, and should not be expected. He does have a better shot than any other player active today, but I have to agree that regression is more likely than continually maintaining such an elevated level of play. That only means that he is more likely to be an excellent player rather than an historically great one.
One might throw a net over every player that got "House Money" via an eye popping multi million dollar - multi year contract - and hit at a 66.6% clip or better.
Your comments on AJ Burnett may prove spot on, but man are throwaway comments like "the Phillies infielders aren't getting any younger" are getting so tiresome. There's not a person on the globe who is getting "younger."

And the left side of the Phillies' infield may not be elite, but Rollins and Asche aren't sieve-like either. I also expect a good amount of playing time at 2nd, SS and 3rd for Freddie Galvis, whose infield defense, I assume, meets with BP's seal of approval.

I agree completely with the take on Papelbon and wish the Phillies somehow could have moved him this winter. But putting Burnett on this list strikes me as "we don't like the Phillies, the Phillies don't like advanced metrics, so we're going to assume that for them, anything that can go wrong will go wrong."