When the Twins signed Joe Mauer to his 8-year, $148 million deal, it was assumed that Mauer would eventually be moving to first base. It seems unlikely that anyone in the Twins organization thought it'd be happening this soon, but the team is announcing that Mauer will make the move after talking with the team's doctors and doctors at the Mayo Clinic.
As sudden as the move may seem, the Twins have been gradually easing Mauer away from catching for awhile. Since his injury-riddled 2011 season, he's been making less appearances behind the plate, starting just 73 games there this season and 72 games in 2012. After making 30 starts at first base in 2012, he made just 8 starts there this year and DHed 29 times. The last time Mauer started 100+ games at catcher was 2010, before his knee issues started to pop up.
Mauer turns 31 in April and it's clearly in the best interest of the Twins to keep him healthy (have you tried watching a Mauer-less Twins game?), but at the same time, it won't be easy for Mauer to be worth what Minnesota's paying him at first base. At catcher, Mauer provided very good defense at a difficult position while being one of the best hitters at his position. At first, his defense will still probably be above average (there are a lot of bad defenders there), but he won't provide the kind of power you'd typically expect from that position. In terms of WAR, you can probably expect a dip in Mauer's numbers, as he's going from a position that provides the biggest boost to a player's value to one that deals the biggest blow in positional adjustment. WAR isn't everything, sure, but it does back the point that Mauer's a more valuable player behind the plate.
With that said, Mauer is still one of the best hitters in baseball. He's going to get on base a ton, no matter where he plays. He has five years left on his deal with the Twins, earning an even $23 million per year for the remainder of the deal. In the end, Minnesota is hoping the full-time move to first leads to more games with Mauer in the lineup (his 147 games in 2012 were a career high, and he's only played in 140 games three times in ten years), balancing out whatever value may be lost by moving him off catcher.
Who replaces Mauer as the team's primary catcher? It looks like Josmil Pinto may get the first shot at the job. The 24-year-old tore through the Twins' system this year, hitting .309/.400/.482 between Double-A and Triple-A before earning a September call-up. In 21 big league games, Pinto hit .342/.398/.566. The fact that he's hit so well likely makes this decision to move Mauer a bit easier.