In the era of 3 man benches, your standard bench players almost have to be a backup catcher, backup infielder, and backup outfielder. Slotting Mauer into a bench spot requires your backup cf, catcher, or ss into the starting lineup. Kepler could start and backup Buxton in CF if needed. Keeping Mauer would mean a bench of Mauer, backup IF (probably Adrianza), and backup catcher. Adrianza or Garver would need to play in the outfield to give an outfielder a day off and force Cave back to AAA. Seems like a pretty weak lineup. That's the real cost of keeping Mauer with a 13 man pitching staff.