I wouldn't replace him. He's a really nice player. He just isn't able to provide what Paul Pierce could. He doesn't have great separation moves, and he doesn't make that little second move to freeze big guys.
If he can get that left foot down and get the ball off the floor he can finish over anyone. He needs to drive on the move because when the defense is set it is too easy for the help to attack his dribble and screw him up. He's cool under pressure so he's a great second option at the end of games, but not a great primary guy.
When Evan Turner is your best one on one scorer, you will not be competing for a championship. We need one cold blooded isolation scorer. We don't need to get rid of Green.