My friends and I have talked about this quite a bit. Lee forced him to alter his throwing motions and mechanics so its going to take a long time before the results started to show, especially if he just started doing it in training camp.
I've played a sport where I had to change my mechanics after playing a certain way for over six years and it took me another year before I had things pinned down to the point where I was playing better. I got worse before I got better. It just takes practice and reps to build muscle memory. With Fitz, he struggled early and is now starting to perform better towards the end of the season after slowly becoming more comfortable with the new mechanics. Now, I'm not saying that he's going to be some elite quarterback, but will likely be average.