I always believed that the 2nd catcher could not get injured (assuming there were no others on the roster), but then had this happen:
http://onlinegames.strat-o-matic.com/game/playbyplay/423795/758My team has two catchers-- a platoon of Frank Duncan and Jim Sundberg. Sundberg started the game. In the top of the 7th, HAL did a double-switch so Duncan would lead off the next inning -- and he got plunked and injured for 7 games, with my backup shortstop, Craig Reynolds, filling in behind the plate.
Oddly enough, in another game that season, HAL put in Reynolds behind the plate without an injury - and he threw out Ichiro stealing in the 9th inning of a tight game!