I've been waiting for this move for a while. I think Fitz just doesn't have the physical tools and surprisingly, the mental side to process things quickly enough or the discipline to make the right decision consistently enough to remain a starter.
I've not been too impressed with Mallet but understand his potential because of his physical tools. The threat of him getting the ball down the field may open some things up at some point but he likely won't have Foster as bad as that injury looked. So, while he has the big arm to get it deep he probably won't have the time to do that until he beats some blitzes and forces defenses to respect him.
If he can make quick, smart decisions and accurately get it to his weapons the Texans will improve on offense. But I haven't seen that from him the little bit that I've watched him. For the Texans' sake, hopefully O'Brien has been coaching him up.