Doc Rivers’ playoff rotation, and especially the starting lineup he wants next weekend, has been more elusive than the bald eagle. The Celtics coach has needed the patience of a bird watcher for this group to come back to the nest. But now that it has, with the core group of Kevin Garnett, Paul Pierce, Jeff Green, Brandon Bass and Avery Bradley as healthy as it is going to be, there’s a next step. Green has to continue his ascent of the last three weeks. The Celtics forward finally proved he can take charge with the Pierce/Garnett combination off the floor. His new challenge is to strike a balance with them. In the past he has been too deferential to the Big Two. Though no one expects Green to morph into Kobe Bryant, Garnett and Pierce are encouraging him to flex that mercurial ego now. “I told Jeff tonight, the nights when he plays with Paul, myself and (Rajon) Rondo, those are the nights when we have the opportunity to give him an opportunity,” Garnett said following the Celtics’ victory at Orlando on Saturday night. The win clinched the seventh playoff seed for the Celtics, who will face the No. 2 Knicks in the first round. If Garnett were a chef, he would be telling Green to raid the buffet. “That’s the time when he should take the shots given to him, though he doesn’t have to take his man one-on-one,” Garnett said. “We have plays for him, and when his play is called that’s a whole other story. When we’re out he has to initiate a lot and the ball is in his hands a lot. That’s when he (creates) matchup problems. “But I’ve always said let us make it easy for you. There are times when I feel he does overwork, and he doesn’t necessarily have to. But the fact that he’s aggressive is a great sign for us. We have to continue to encourage him to be that.”