When he returned from his injury, he was throwing overhand. That had nothing to do with the injury, it was just that the staff finally was able to convince him that that was the correct way to throw. And he was throwing very well, for about a week. But then he reverted back to sidearm. One explanation I heard was that he really wasn't fully recovered, hadn't completely regained his arm strength, and had to compensate by varying his throwing motion.
So all this talk about moving him to first base is silly. Let the guy recover completely and get back to throwing overhand. He'll be fine. He is the best thirdbaseman in the game and it would be a huge waste to play him at first. (What are you going to do with Morse, then, if Zimmerman goes to first?)