Joe Mauer‘s accomplishments as a catcher through age 30 are nearly unmatched in the history of baseball. As a 23-year-old he became the first AL catcher to win a batting title, the first catcher in either league to win a batting title in six decades, and the first catcher in MLB history to lead all of baseball in batting average. As a 25-year-old he won another batting title and as a 26-year-old he became the first catcher in baseball history to win three batting titles.

In that age-26 season he hit .365 with a .444 on-base percentage and .587 slugging percentage on the way to being named MVP. He was the first catcher in baseball history to lead the league in batting average, on-base percentage, and slugging percentage–the sabermetric triple crown–and his .365 batting average was the highest by any catcher since 1901. Mauer also made three other strong runs at batting titles, finishing third at age 27, fourth at age 29, and second at age 30.

Overall from debuting as a 21-year-old in 2004 through his age-30 season in 2013 he totaled one MVP award, three batting titles, three Gold Glove awards, five Silver Slugger awards, and six trips to the All-Star game while hitting .323 with a .405 OBP and .468 SLG, logging 8,000 innings at catcher, and throwing out 33 percent of stolen base attempts. Add it all up and Mauer produced the fifth-most Wins Above Replacement by any catcher in MLB history through age 30:

CATCHER WAR Johnny Bench 63.9 Gary Carter 55.5 Ivan Rodriguez 50.4 Ted Simmons 44.8 JOE MAUER 44.2 Mike Piazza 41.5 Mickey Cochrane 40.7

Unfortunately he’s no longer a catcher, being forced to move out from behind the plate following an August 19, 2013 concussion that ended his 2013 season and left him with symptoms months into the offseason. Mauer was given a clean bill of health for 2014 and switched positions, going from one end of the defensive spectrum to the other, but now he’s a 32-year-old first baseman who looks like a shell of his former self.

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Mauer got off to a slow start last season, which is never unexpected for a player returning from a season-ending brain injury. When he started doing his usual .300-hitting, walk-drawing thing in June–and for the most part kept that up through the rest of the season despite being interrupted by a strained oblique muscle–it looked like Mauer was fully recovered from the concussion and ready to resume being an on-base machine in 2015.

Optimism was further fueled by reports of Mauer working with hitting coach Tom Brunansky to be more aggressive at the plate and pull more pitches in an effort to unearth more power. There were some signs of progress early on this season, as Mauer turned on pitches and lined balls into right field and right-center field more often than he did previously, but his overall production has fallen even further from his underwhelming 2014 production.

Mauer was having a typical Mauer season at the time of the concussion, hitting .324 with a .404 OBP and .476 SLG compared to his nearly identical .323/.405/.468 career mark from 2004-2012. He has not been the same hitter since, whether due to the lingering effects of a brain injury or to the normal effects of aging, the latter of which are often more rapid for longtime catchers. Mauer has hit .272 with a .348 OBP and .366 SLG in 178 games since the concussion.

AVG OBP SLG SO% BB% ISO Pre-concussion .323 .405 .468 11.2 12.2 .146 Post-concussion .272 .348 .366 17.6 10.5 .095

For a player whose hitting revolved so much around eyesight, handeye coordination, and reflexes rather than brute strength the possibility that the concussion still hurts Mauer by robbing even a fraction of those skills seems plausible. Whatever the case, the Twins now have a 32-year-old first baseman with more than $80 million remaining on his contract who has just narrowly managed to be an average all-around hitter for the past 178 games.

Mauer has always lacked power, or at least power relative to his 6-foot-5 frame and unrealistic expectations for how he might develop into a perfect, flawless player, but even his modest power has vanished. Once good for a ton of doubles and 10-15 homers per season — including 28 homers in his MVP-winning 2009 campaign — Mauer has just six homers in 178 games since the beginning of 2014.

Isolated power is a statistic that subtracts batting average from slugging percentage to show how much raw power is present. Mauer’s isolated power since the concussion is .095. By comparison, his isolated power from 2004-2013 was .146 and dipped below .115 just once. He has essentially lost half of his power since the concussion and the result is the type of player–a punchless singles hitter–that Mauer’s critics often unfairly labeled him as throughout his career.

In the past two seasons MLB hitters as a whole have an isolated power of .140 and first basemen have a collective isolated power of .185, which means Mauer has produced 31 percent less power than the average hitter and 48 percent less power than the average first baseman. To get a sense for just how rare it is for a first baseman to have such little power here’s a list of the lowest career isolated power totals by regular first basemen in the past 20 years:

FIRST BASEMAN ISO Daric Barton .118 Casey Kotchman .125 James Loney .129 Doug Mientkiewicz .134 Conor Jackson .136 Scott Hatteberg .137 Sean Casey .145

Mauer’s pre-concussion isolated power of .146 would have been among the worst by regular first basemen since 1995, but his post-concussion isolated power of .095 is basically unheard of at the position. There’s a subset of left-handed-hitting, slick-fielding first basemen like James Loney, Casey Kotchman, and Doug Mientkiewicz that always seems to have a couple starters across MLB each season, but even those guys typically manage an isolated power in the .130 range.

Mauer going from mediocre power to non-existent power would be somewhat easier to live with if he were still hitting .325 with tons of walks, but even that has changed. Mauer’s strikeout rate was on the rise even before the concussion and since returning from the brain injury he’s whiffed in 18 percent of his plate appearances compared to 11 percent from 2004-2013 and less than 10 percent in some of his best seasons.

Whereas most players sacrifice strikeouts in the name of making harder contact Mauer’s batting average on balls in play this season is a career-low .302 and his .328 batting average on balls in play since returning from the concussion is 20 points below his career mark. He’s putting fewer balls in play and fewer of the balls he does put in play are dropping for hits, which is a very bad combination.

After constant complaints that he’s too passive at the plate–which, to be clear, always struck me as absurd for a player hitting .325 and winning multiple batting titles–Mauer has indeed become more aggressive. He’s swung at 44 percent of pitches this season after never cracking 40 percent in a season previously, which includes career-highs in swings on pitches both inside and outside of the strike zone.

Mauer has pulled 31 percent of the balls he’s put in play this season, compared 27 percent for his career. However, his rate of “hard hit” balls is 27 percent, which is the second-lowest mark of his career. His rate of “soft hit” balls is 16 percent, which is the second-highest mark of his career. He’s tried to change his approach, both in terms of being more aggressive and pulling more balls in the air, but it’s not clear that the net result to those changes in a positive one.

Basically everything about Mauer’s hitting has gotten worse since he suffered the concussion. He’s striking out more and walking less. He’s swinging through more pitches and hitting pitches softer when he does make contact. Pitchers are also throwing him more strikes, in general, which is to be expected at some point when the league starts to grasp that the .325 hitter has now been a .275 hitter for 18 months.

The only saving grace, at least so far this season, is that Mauer has been incredible with runners in scoring position and in high-leverage situations. That’s actually nothing new, but the difference now is that Mauer has thrived in those spots while not being good in other spots. He’s hitting .432 in high-leverage situations and .397 with runners in scoring position, which is how he’s on pace for nearly 100 RBIs and leads the Twins in Win Probability Added despite just two homers.

It seems clear that the combination of a normal aging curve plus logging 8,000 innings behind the plate through age 30 and multiple significant injuries, chief among them a concussion in August of 2013, has led to Mauer being a shell of his former self at age 32. Because he’s a great athlete with amazing hand-eye coordination who ranked as one of the best all-around players in baseball for a decade he’s still able to be a reasonably productive player in his diminished state.

However, the shift from catcher to first base was going to take a big chunk of his value away even if his offensive performance stayed constant and instead it has deteriorated rapidly. Perhaps he’ll have a resurgence like former Twins teammate Justin Morneau and other prominent hitters have had several years after being derailed by a concussion, but in the meantime he ranks as a below-average player at his position after a decade of ranking as the best player at his position.

The hope with Mauer’s move to first base was that getting out from behind the plate would help him stay healthier and more durable, enabling him to up his production enough to be one of MLB’s better-hitting first basemen in the Joey Votto mold. Short of that, a reasonable expectation was that Mauer’s hitting would remain unchanged from his catching days, turning him into a very good but not spectacular offensive first baseman in the John Olerud mold.

Instead it’s been a worst-case scenario. He’s hitting worse than ever at a position with the highest bar offensively, derailing Mauer so far off course from a Hall of Fame path at catcher that he fits into the Mientkiewicz/Loney mold at first base. It’s a sad change and one that’s been difficult for everyone to wrap their heads around, but based on the information we have about Mauer since his concussion the odds of him returning to the 2004-2013 version seem slimmer by the day.

It’s important to remember that Mauer struggling after age 30 doesn’t take away from what he accomplished through age 30. He was a truly elite catcher, a .325-hitting on-base machine with a strong arm who ranked as one of the best handful of players at his position in baseball history. At the same time, Mauer being a Hall of Fame-caliber player through age 30 doesn’t change the fact that he now needs significant improvement simply to be an above-average player at age 32.