Perkins has not been giving the Thunder much this season. His PER for/against is comparable to to O'Neal's, and his net +/- is much worse than O'Neal's.
I think if he were on this Celtics team he might be giving us 20-25 minutes a game, but at a performance level similar to what JO does. The upgrade would be at backup center, where JO would be better than Stiemsma, but even there I don't see the difference as being all that dramatic.
Having Perkins might be worth 3-5 extra wins over the course of the year.
The difference would be greater if JO were to get hurt, obviously. I think that is the real risk with our team right now, in terms of missing Perkins.