If I had to pick three guys, they'd probably be the ones you listed: Rondo, Green, and JO.
Rondo absolutely has to dominate his matchup. I expect each of Lebron, Wade, and Bosh to outscore their Celtics counterpart, sometimes by a healthy margin. It will be on our other players to make up that margin, and the biggest factor in doing that will be at PG. Rondo needs to punish Bibby.
Green will be important in helping keep Pierce fresh. In the past, Paul has struggled offensively when asked to do a ton defensively on Lebron. If that's the case here, we're going to need Green to come in and be aggressive in 10 to 15 minutes per game. He's going to have to work hard on defense, too, so that Lebron doesn't explode when Paul is on the bench.
We need JO to do what he did in the NY series -- block shots on drives to the paint -- but we also need better rebounding from him. How well he defends + rebounds is going to be crucial here. Hopefully, Shaq can come back to give JO some support; I don't like the idea of Krstic or Baby trying to cut off penetration.