More to the general point about players only being focused on their job on the field....
Then why would Joey Votto go to the DL with anxiety issues? I think it is pretty clear that he was affected by those issues to the point where it was affecting him on the baseball field. And if one of the best players in the game can be affected so deeply by anxiety it stands to reason that others can as well-- to all different degrees.
If Nelson Cruz is ever in a clinching WS game, I wouldn't be surprised if he tightened up. I wouldn't be surprised if he got in his own head and started stressing about another catchable line drive coming his way.
There is a ton of standing around in baseball. Lots of time to think and psyche yourself out. In that way it is similar to golf--- and everyone knows about the "yips" in golf when a pro suddenly can't sink a three foot putt. Or when a guy like Chuck Knablauch suddenly can't throw the ball to first base anymore without injuring a fan in the front row.
Take those individual issues and anxities and apply them to a team--- I think it is a very reasonable thing to at least ponder.