Jeff Green has been playing out of position his entire career in OKC/SEA. In Boston he is going to be an athletic 6th man who can score and defend several positions coming off the bench, which is something that the Celtics have not had.
People are giving Perkins way too much credit for the success that the Celtics have had against Orlando, but if you look at the game stats (they were listing them on the radio this morning, and I haven't had a chance to find them online, so I'll take their word for it) Dwight has gotten his points and rebounds against Perkins in almost every matchup. The Celtics are fine with him doing his thing against them, what they don't do is double team Howard.
Everyone is saying that Perkins was the defensive heart and soul of the Celtics, which is insane. Kevin Garnett's resurgence this year has keyed the Celtics defense. With or without Perkins in the line-up.