I don't know if you have ever played defense on football or not, but it is anything but easy. Changing teams is easy because it is just a different jersey, but changing schemes is very very difficult. Not to make excuses because he did have a very subpar season, but he still needs time. there is very little thinking time on the feild especially at corner so you teach yourself habbit and that isn't something that can be immediately changed. So with all due respect to your evaluation of David I believe you are incorrect.
Edit: If he were a subpar athlete then your assertion that they hide his flaws in Ind. would be correct, but that is not the case. He just learned a different system.