Just wanted to make this point before the free agent frenzy starts. Jeff Green is not James Posey. He's versatile but he's not equally suited to all the roles that he has played. I like him at small forward, I'm a lot less fond of him at power forward. It is a little like a switch hitter that hits .300 from the right side and .220 from the left. At some point, you just have to play to your strength.
I get that in theory he can out-quick opposing power forwards in a small-ball type of lineup that Doc has found some occasional success in, but I don't see it as a very good long term strategy. On offense, I guess you can sometimes catch someone sleeping, but what really worries me is on defense.
What changed were his defensive results. According to 82games.com, opposing small forwards mustered only a 7.9 PER against Green with Boston and a 12.2 PER against him with Oklahoma City; the problem was that opposing power forwards shredded him for a 21.6 mark. Similarly, Green's Synergy stats with Boston were strong, whereas with the Thunder they were awful. All of which points to the fact that Green is much better at guarding 3s than 4s. He won't be a great player at either position, but he can defend the wing well enough to be a solid rotation player even if his offensive output continues to be relatively pedestrian, and that explains Boston's logic in acquiring him.
Now, I guess his minutes have to come from somewhere, and despite what Danny floated as an idea last year - Jeff Green is not starting over Paul Pierce anytime soon. So I can see him getting spot minutes at the 4. But I don't have to like it. In fact, I'm hoping he "gets his minutes" more in terms of giving Pierce strategic rest throughout the year. I wouldn't mind seeing Pierce get the occasional 20 to 25 minute night - in particular if Green is playing well.
Now, I'm perfectly willing to be proven wrong. If Green re-signs and learns his defensive rotations and really makes a great effort to defend both positions to the point where he's not hurting the team defense, then great. He can play wherever he wants. However, I'd just feel a lot more comfortable if the Celtics signed a few young, healthy big men and gave JaJuan Johnson some minutes at power forward so we don't have to resort to the plan G at the 4.