By Bloomberg Sports //

Last season’s AL MVP, Joe Mauer, has seen his home run numbers drastically decline this season as compared to his 2009 level. Mauer hit a career-high 28 homers during his MVP campaign, but so far this year he has hit only two.
Using Bloomberg Sports’ statistical tools, we can see that Mauer’s home run output both in 2009 and in 2010 were uncharacteristic, and that his power numbers should regulate somewhere in between the two.

First, it must be mentioned that Mauer’s OPS in 2009 was unusually high for his career, and a neutral observer might conclude that 2009 was a fluke. Others could argue that this surge had to do with natural age progression, as he reached the age of 26, a milestone at which many players begin to peak. Thus, despite Mauer’s current career OPS of .887, it could be argued that his 1.031 2009 OPS would have some staying power. Mauer’s monster season prompted the Twins to hand him an eight-year, $184 million contract extension in March.

Mauer’s two homers this year-to-date have thus raised concerns in Minnesota. We can point to his fluctuating home runs per flyball rate as a cause of this season’s power outage – as well as Mauer’s 2009 outlier season. From 2005 to 2008, that rate ranged from 6.5% to 10.8% — league average typically hovers around 10%. However, in 2009, Mauer’s HR/FB rate jumped to a stratospheric 20.4%. Thus maybe this shift, rather than indicating anything in particular about Mauer’s game, indicates that a lot of Mauer’s power in 2009 was the result of a statistical variation. If this dramatic increase had been accompanied by a drastic change in body type it would be understandable, but Mauer’s body did not noticeably change.

This season, Mauer’s homerun per fly ball out rate has regressed to just 5.7%, a career low, but also closer to the pre-2009 range. The numbers clearly point to 2009 being an outlier in this respect. Granted, a couple of unmentioned variables might be pulling down Mauer’s HR/FB rate this year. One, the Twins’ move to Target Field from the Metrodome might be affecting his power numbers, especially through the early, colder-weather months of the season. Second, pitchers might be attacking Mauer differently this season, following his ’09 power outburst. Let us explore these two possibilities.

Mauer’s new home, Target Field, has been the third-worst ballpark for home runs, in front of only Citi Field and the Oakland Coliseum, according to ESPN.com’s MLB Park Factors. However, through just over one-third of the season, it is tough to say that this phenomenon is reliable or that it will remain constant. After all, it typically takes three years before you can properly trust a given stadium’s park factor. Meanwhile, Mauer’s previous home, the Metrodome, played as roughly home run neutral. The dimensions of the two fields are very similar, so expect Target Field to be less home run-stubborn than it currently is, especially in the warmer summer months. To date, Mauer’s OPS is 80 points lower at home than away.

Maybe pitchers are attacking Mauer differently too. This argument could potentially explain some of the catcher’s power struggles. Pitchers, in fact, have been unwilling to throw him fastballs in certain counts, and seem to have replaced them with changeups, and occasionally curveballs. In 2009, on 0-1 and 2-2 counts, Mauer saw a majority of fastballs. However, this season, in the same counts, he’s seen a fastball only about one-third of the time. This change by the pitchers seems smart, as Mauer had a lethal 1.077 OPS against four-seam fastballs from 2006 through 2009. Meanwhile, he now sees almost three times as many curveballs on the first pitch and in 2-2 counts, according to Bloomberg Sports tools, when compared to 2009. This adjustment by pitchers seems appropriate, as Mauer had a .483 OPS against curveballs dating back to ’06. Lastly, Mauer has seen more changeups in 1-2, 2-1, and 2-2 counts, although he has fared well against the changeup in his career, so this adjustment should have had no effect.

Target Field’s low home run rate and the new approach by pitchers may be hurting Mauer’s home run numbers. But the statistical variation in his HR/FB rate also helps explains the drastic difference between 2009 and 2010. That rate suggests that Mauer’s MVP-type numbers may have been affected by a statistical outlier, and that fans and teams may have to reassess their expectations for Mauer’s power numbers. In regards to how pitchers are approaching Mauer, it seems unlikely that the recent adjustments can explain this year’s low home run total, as he has been a top player in the league since 2004, and pitchers have been adjusting to his tendencies every year. Meanwhile, Target Field has been playing like a large shopping mall – but it does not explain Mauer’s low road home run total, or the fact that he has yet to hit any homers at home.

Expect a middle ground to emerge between the home run binge Mauer showed last season and the drought he’s experienced in 2010.