Well when you play rookies 38 minutes a game in their rookie year, they usually tail off at the end of the season because their bodies aren't accustomed to that workload.
So, if anything, you can say that his body is being worked up in order to sustain 38 minutes a game.
Besides, if the Cavs are getting blown out, why keep him in the game? Plus, he was hurt for a few games, and they barely played him in April because it wasn't necessary.