This is actually a great suggestion. One of things that gives the User such a huge advantage over the CPU is that User can keep undisciplined players from making so many poor decisions, and therefore make them so much more effective than they are in real life.
I know that a strong appeal of using any team is to have players like JR Smith and Westbrook live up to their potential by getting them to always play under control and more effcient. IMO, things are more interesting if there is a consequence for, "reigning a player in" and making him play against his nature/personality.
For example if the User eliminates all the 3 pointers from Josh Smith's game, he would obviously be an exponentially more efficient player, and his team would benefit from this change too, of course. So he's "doing what the coach wants", but he should be unhappy, and other parts of his game could/should suffer, and he might want to be traded as well.
Just something to think about.