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Pedro Strop was a utility infielder who spent three years in short-season ball and never slugged as high as .350 before the Rockies decided, in 2006, to try his live arm on the mound. In his first try as a pitcher, he struck out 22, and walked just two, in 13 innings. But the Rockies released him two years later, and the Rangers picked him up, waited him out, and eventually promoted him to the big leagues. He had an ERA over 7, with 22 walks in 27 big-league innings, when they traded him to Baltimore as the non-famous half of an August waiver-period trade for Mike Gonzalez.

That was a year ago this week, and Strop has been far more significant in this year’s pennant race than Gonzalez ever was with Texas. He has the second-best ERA in the American League (minimum 30 innings), and among relievers, his win probability added is sixth, ahead of all but four closers. He has the fifth-best groundball rate in the majors, a fastball that averages 97 mph, and no platoon split worth worrying about. The Orioles entered play Sunday tied for a Wild Card spot, though their playoff odds were just 13.4 percent. It’s probably no exaggeration to say that, without Strop's work this year, those odds would be close to zero.

You could say the same about the Orioles closer Jim Johnson, and about the side-arming set-up man Darren O’Day, who rank first and 10th in WPA. You could absolutely say that about the Orioles' bullpen, which has the league’s third-best ERA, and which has been the most obvious reason that the Orioles have gone 23-6 in one-run games, which has been the obvious reason they have managed to get this far despite a similar run differential to the Royals'. When a team's record strays from its run differential, the simplistic assumption is that it's about the bullpen, or luck. But from a team-building perspective, that might be redundant. A good bullpen is ephemeral, and the correlation from one year to the next is basically insignificant.

That’s not hyperbole. The correlation between AL teams’ bullpen ERAs in 2011 and 2012 is an insignificant .06. The correlation between teams’ bullpen ERAs in 2010 and 2011 was actually negative, slightly. That oversells the point a bit: the correlation varies greatly from year to year. But over the past 10 years, the average year-to-year correlation is a very weak .18. (Teams’ starting-pitching ERAs, by contrast, are much steadier. The correlation between 2011 and 2012 is .74, and the average correlation over the past decade is .58.) Considering that I’m using a stat that isn’t even park-adjusted—that is, simply playing in the same ballpark should provide some consistency from year to year—this should be discouraging news for Orioles fans hoping to relive the O’Day-to-Strop-to-Johnson magic next year.

The AL's top three bullpens last year—Yankees, Angels, Mariners—rank seventh, 12th, and fifth in ERA this year. The top three bullpens this year—Rays, A’s, Royals—ranked sixth, seventh, and eighth in the AL last year. The Orioles were 13th last year.

Using the Angels as an example, we can see how bullpen success one year can be so fleeting. The Angels' bullpen in 2011 was a testament to Mike Scioscia’s ability to find roles in which marginal arms could succeed. That’s what I would focus on if I were face to face with Mike Scioscia, and I wanted him to like me. But the other way of saying it is that the Angels' bullpen last year comprised:

  • A groundball specialist who far, far, far outperformed his peripherals thanks to a .204 BABIP with runners on base;
  • A veteran who had the best ERA of his career but was shedding strikeouts at a troubling rate;

  • A rookie closer who was very effective, but who had been a mediocre Double-A reliever one year earlier;
  • A generally effective strike-thrower whose perceived flaws were amplified by high-leverage situations and who, therefore, didn’t seem to have his manager’s confidence;

  • A 36-year-old signed for decent money as a free agent, whose regression was covered up by a low BABIP;
  • Some Triple-A starters who were occasionally brought up to provide depth;

  • Fernando Rodney, mercifully unavailable for half of the year, and (haha) not re-signed.

The low-BABIP specialist, Bobby Cassevah, has been a disaster this year; even after a demotion to Triple-A, his ERA in Salt Lake is 6.65. The veteran, Scott Downs, has similar peripherals as in 2011 but an ERA twice as high. The rookie closer, Jordan Walden, lost his job, got injured, and has shown the unreliable control he had in Double-A. The strike-thrower, Rich Thompson, was released. The 36-year-old, Hisanori Takahashi, dramatically improved his strikeout and walk rates, but gave up loads more runs as his strand rate approached zero. The Triple-A starters are all gone, replaced by other Triple-A starters. The holes left by the absences of Cassevah, Thompson, and Rodney have been filled by a quartet of right-handers (call-ups and free-agent signings) who have been competently uninspiring. And, saving the group from total ruin, Ernesto Frieri joined the unit.

In summary: there’s a lot of personnel turnover; there are small-sample successes that get un-small-sampled; there are BABIPs; there are fluctuating strand rates; there are injuries; there is aging; there are changing roles and short leashes; and there is, as always, all the periphery stuff (chemistry; catchers; confidence; concealed injuries, perhaps) that is either unmeasurable or only barely measurable. It's not that we should have expected the Angels' bullpen to falter; it's that we shouldn't have expected anything, despite the 2011 success.

The Orioles' bullpen success looks just as fragile. Strop, for instance, isn’t Cassevah, and he isn’t Walden, but he's not a bad mash-up of the two, with a short record of success, an unexciting strikeout rate, loads of walks, loads of grounders, and all the traditional markers (low HR/FB rate, low BABIP, high strand rate) of luck. Jim Johnson isn’t Downs, but his strikeout and walk rates do a good impression, and his extreme groundballing won’t always come with a .235 BABIP.

Putting together a working bullpen has to be one of the most fulfilling parts of running a team. Every one of the pitchers in the Orioles’ bullpen is a find, and every one is a story. Troy Patton was part of the return for Miguel Tejada, blew his top-prospect status as a starter, and is now a very good lefty specialist. Darren O’Day spent two years absolutely dominating in Texas and then undid it all with a lousy postseason and worse 2011 season; the second time the Rangers went to the World Series, O’Day was left off the roster and the Orioles grabbed him off waivers a couple weeks later. Luis Ayala had a 2.09 ERA, in New York, but because we’re all so smart now and his FIP was basically twice that number, nobody bought it for a second; the Orioles got him for less than a million bucks, with a team option thrown in, and here he is doing it again. Matt Lindstrom was pitching for his fourth team in four years and has lowered his ERA each time. (And now he’s pitching for his fifth team in four years. The Orioles traded him for Joe Saunders on Sunday.) Jim Johnson was a fifth-round pick who got to close only because the Orioles were supposed to be too lousy to require a big-money closer. Lo and behold, the surprising Johnson has keyed the Orioles’ success, and the surprising Orioles gave Johnson so many save opportunities that got to be an All-Star.

And then there’s Strop, whose acquisition is the perfect microcosm of this article’s point. Strop was an apparently lousy reliever traded for a pretty good reliever. The pretty good reliever, Mike Gonzalez, didn’t do squat. The lousy one turned into a bullpen ace, for a year at least. You just can’t predict bullpens. That’s what makes it all fun, and that’s what makes the Orioles’ 2012 season possible.

And unrepeatable.