So, .239/.326/.375. That’s not very good. It might be acceptable for a good defensive middle infielder, or perhaps a catcher. It might look good for an outfielder in 1968, when the league hit worse than that as a whole. That’s the line, however, of a 24-year-old center fielder in the first half of 2009, and it represents one of the more disappointing performances of the season to date. It’s also the first-half line of the player who could well be the best in baseball in the second half.

B.J. Upton, coming off of a fantastic 2008 postseason and subsequent shoulder surgery, was supposed to take a big step forward this season. The raw talent that made him the second overall pick in the 2002 draft was being leavened by experience, not all of it successful. Upton had to fail as a shortstop and as a second baseman and had to flop in his first two major league stints before he broke through in his third season. It was during that season that the Rays made the critical decision to move Upton out of the infield, where he’d been terrible his entire professional life, and into center field. With his great speed and baseball instincts, Upton took to the change. The move turned him from a mediocre player to a very good one in a season when he hit .308/.368/.508 in mostly everyday play.

In 2008, Upton made tremendous strides across the board. He improved his play in center, cut his strikeout rate, improved his walk rate, and stole twice as many bases, albeit without improving his success rate. On May 1, he was batting .294 and slugging .441 when he suffered a debilitating shoulder injury. He played through it the rest of the season, but wasn’t the same hitter, batting .268 and slugging .392 the rest of the way. He hit just six homers in his final 520 plate appearances, a far cry from the power he’d shown in 2007. Even at that, his dramatic improvements in other areas showed him to be a coming star, an MVP candidate in the making.

All of those skills are still present, hidden behind that terrible line. As much as we sometimes infer that player development is a linear process, it’s not. Players, even great ones, bounce around, they get unlucky, they pick up bad habits. In Upton’s case, a look at his season to date shows two notable trends that have affected his performance line, elements that he is likely to correct as we head into the second half.

The first is that Upton is hitting more fly balls than ever before, and it’s not close. Upton hits fly balls in 42.0 percent of his at-bats, versus a career mark below 34 percent coming into this season. Upton has lower rates of ground balls and line drives as a result of this. If you’re wondering where the missing 60 points of batting average are-as Upton was expected to be a .300 hitter-that’s your first culprit. Batting average is lower on fly balls than on other batted-ball types, so Upton’s average on contact has dropped from last year’s .360 to this year’s .340. His batting average on balls in play shows that same drop, from .344 to .317.

Upton has also taken a step backwards in terms of making contact, which was his most dramatic area of improvement in ’08. After pulling his K rate down from 32.5% to 25.2%, it’s back up this year to 29.9%. That’s the rest of the missing batting average. Upton may be striking out more as a side effect of his ever-increasing patience at the plate. He saw 3.7 pitches per plate appearance in 2006, a bit fewer than 4.1 in 2007 and 2008, and is a sliver above 4.1 in 2009. Deeper counts will up your strikeout rate, but should also result in more walks and, for good hitters, more opportunities to hit in good counts. Upton isn’t seeing the benefits yet, however, because despite the deeper counts, he’s not necessarily making better decisions at the plate. He’s swinging at more balls out of the zone than he did last year (up 2.5%) and making less contact when he does (down 14.4%).

There’s a term I probably use too much called “consolidation year.” It’s what happens when a player is integrating learned skills with innate tools, a process that can lead to short-term degradation in performance. Upton has shown most every skill you can ask of a baseball player at one time or another. He’s hit for average and for power, and a healthy shoulder has enabled him to display a bit more of the power we’ve expected from him. He’s drawn walks. He’s stolen bases. He’s shown range in the outfield, and he’s displayed very strong arm. Even in a disappointing season, he’s posted the best stolen-base percentage of his career, and is on pace for a career high in steals.

Whatever wildly optimistic numbers I would have hung on Upton four months ago are obviously out of reach. However, because of his age, his ongoing development, his freakish tool kit, I think he’s going to be that player for the last 10 weeks of the season. Upton is going to hit .330/.400/.530, going to steal at least 20 bases at a high rate of success, going to be a plus defender in center field, going to cut his strikeout rate and hit more line drives.

There’s an element of faith in these statements-after a big June made it look like he’d found himself, he started July by striking out in one of every three plate appearances while batting .175-but I look at Upton’s season to date as I look at Tiger Woods’ 2003-04. Woods was the best player in the world by the end of 2002, but at 27 had to rebuild his swing to become an even better player. For two years, Woods didn’t win a major while changing his game, and came out on the other end as the greatest player ever. Upton hasn’t had the success in baseball Woods had had in golf, but the changes to his game in ’09 reflect a player trying to get better, trying to maximize his skill set. He gets there now, and will be the best player in the American League down the stretch, and then the league MVP in 2010.

(Information from Fangraphs was used extensively in this article.)