I noticed he hasn't been the same player after the break as well and I think you can attribute it to a few things:
1) He is getting a little tired. This is the first time he has played a full NBA season (obviously) and he has been playing 110 mph the entire time. He'll learn to play in spurts like every great does and when to get his rest and take care of his body.
2) People start to figure you out. They have the scouting reports, they have the statistics, they know to keep him from the basket and frustrate and bang with him. It's earing on him and getting to him.
3) Skill set. He will most likely gain better footwork through his career, a slightly better jumper, better 1/2 dribble moves, and a better, more calm and collected back-to-the-basket game. Until then, he will still have to rely on running the floor and jumping over people, which will be there at times, and other times not.