That's a problem solved easily enough. When Herrmann is ready, they just drop/demote one of the other guys - and that's if the Twins don't call up a 3rd catcher, which I can also see happening. I can argue it's wise two different ways...
1) When the primary catchers for the major league team are Mauer & Doumit, two guys with SIGNIFICANT injury histories, an organization should probably be cautious about discarding catchers.
2) Herrmann had a monster AFL, but it certainly isn't clear from his numbers that he mastered AA. Or at least, with a .258 BA and a .392 SLG, it looks like he could still learn a few things there.