Back in 2013, Joe Mauer and Carlos Gonzalez were basically on top of the world. Mauer was near the beginning of his mega-deal with the Twins, and Gonzalez was in the middle of a multi-million dollar extension himself. Despite limited playing time, Mauer and Gonzalez managed to place in the top 30 of total fWAR among those players with at least 400 PA. By fWAR/600, Mauer and Gonzalez were 13th and 10th, respectively. They were legitimate stars who were a bit limited in playing time due to injury. Those injuries were harbingers.
It's now 2015, and both the Twins and Rockies are now saddled with big money contracts to players who have fallen far from their star status. As of this writing, both players have negative WAR ratings. Mauer ranks near the bottom among first basemen, and Gonzalez similarly finds himself near the bottom among right fielders. Their signature skills* have declined so much that each player is now playing worse than random AAA players.
*Warning: debatable cherry-picked stats incoming
One might say that Mauer's defining characteristic was his plate discipline. One might also say that Gonzalez's defining skill was his power. Both skills are basically gone, in a way.
For Mauer, it's not necessarily that his plate discipline skills are gone. Yes, his swing rate on pitches outside the zone has increased, but his rate of strikes seen has also increased. Indeed, Mauer's ability to make solid contact has decreased. His rate of grounders and popups is up, his rate of home runs per fly ball is down (more or less), he's pulling the ball more, and his rate of soft contact is up. Pitchers are no longer afraid to challenge Mauer. They're throwing more strikes, and Mauer has responded by being more aggressive across the board. Unfortunately, a Mauer not feared for his spray ability is a Mauer who can't take advantage of his primary skill. Decline in other areas has rendered Mauer's strength irrelevant. And that's sad.
For Gonzalez, it could be a similar story, but kind of in reverse. Gonzalez's plate discipline has never really been a strength; he usually runs low walk rates and high strikeout rates. His ability to smash the ball, even by Coors standards, kept his offense afloat.
This season is different. Gonzalez is showing plate discipline. His walk rate is the highest of his career, and his strikeout rate is the lowest of his career. He is being far less aggressive this year compared to last year, but like Mauer his rate of pitches in the zone is up. Gonzalez is having difficulty putting the ball in the air this season; his 51.4 percent grounder rate is the highest of his career. His rate of hard hit balls is down. It would seem as though Gonzalez's ability to make solid contact is also on the decline. Like Mauer, perhaps pitchers are less afraid of him. In response to his strength being in decline, Gonzalez has attempted to strengthen a weakness. Unfortunately, hitting the ball on the ground does no good in Coors. If Gonzalez could elevate the ball more, he could salvage a good season. For what it's worth (not much), Gonzalez has raked in his 27 June PA, during which he has a 36.8 percent ground ball rate.
While their offenses have been decline, the two players have slid down the defensive spectrum. Mauer, of course, used to be a catcher. 2013 was his final year in that position. He is now a first base/DH fellow. Mauer's new level of offense is nowhere near sufficient at such a position. Mauer went from the position that forgives weak offense the most to the position that forgives weak offense the least. Mauer appears to be an acceptable first baseman, shown by his 5 DRS and 3.4 UZR values over his career at first base, but his offense will not suffice.
Gonzalez played some center field as recently as 2011, but he logged more time in left field in 2012-2014. Gonzalez had mixed numbers at that position; 2012 and 2014 were negatives by DRS and UZR, but 2013 was quite positive. Inside Edge numbers didn't show anything terrible. In fact, they were quite good. However, Gonzalez's 2015 innings have all come in right field. Corey Dickerson and Charlie Blackmon, through their play, have demanded more playing time.
It's entirely possible that Gonzalez and Mauer have reached similar points in their careers. No longer able to man the defensively challenging positions due to some combination of age and injury, the two players have found themselves down the defensive spectrum. Also likely due to some combination of age and injury, each player's ability to strike fear into the hearts of pitchers through his batted ball distribution is gone. Gonzalez has compensated by trying to be more selective; Mauer was already selective. Selectivity will get them only so far if pitchers are not afraid to throw strikes.
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