The reason? Torn meniscus. Not a biggie, of course, but also not the sort of thing that will help him in his efforts to avoid being moved to the DH slot to make way for Jesus Montero.
And it seems that, despite the efforts of some to set this up as a showdown, Posada is fairly cool with that:
“Cashman told Posada that he should come in preparing to catch, but they are going to give Montero a real shot,” the person said. “And Cashman said that Posada should prepare to D.H. a lot” . . . “Posada said great and he was willing to do it . . . He was happy to know what the situation is.”
So there doesn’t appear to be much controversy, and unless Montero just face plants next spring, Posada will likely be the team’s DH.