Kelly is going to need most of his time up front to evaluate his existing players. He then needs to develop a system that will work for the players he is going to keep. It's going to be tricky with FA's because you don't know if they are really "all in" yet. Therefore, Kelly and the players really only have the post draft camp and the OTA's to figure out who gets it and who doesn't.
You can't wait until TC. That's when you need the reps to make sure you can run this stuff. Anybody's who's not bought in at that point should already be gone. Does that make sense?