Offensive End ~ Because he has no left hand. Defenders who take it away make him passive, because he's not a good enough shooter to make guys pay for backing off and shading him left. He misses a few shots, then gets passive. I think if he worked on driving in a more diverse capacity, he'd benefit greatly.
Defensive End ~ It gets a little scarier here. His effort as a man defender and rebounder is oddly poor. In short he has the talent and size to be above average at both, but he doesn't focus every time down the floor, rendering him well below average at each. I'd argue he's the worst man defender on the team, and he certainly shouldn't be.
Bottom Line ~ Much like the Celtics as a team, I think good defense would foster good offense for Green. When he's doing his Ole! routine on the defensive end and defensive glass, he looks equally passive on the offensive end. I hope Doc is preaching defense first to Green in hopes of getting a more consistent effort from him every night...