Given the decline in Jermaine O'Neal's defensive abilities (DPOY candidate to good defender) and his rebounding (good-to-very good rebounder to slightly below average rebounding).
Given those marginal defense/rebounding contributions ... negative offensive contribtions can negate all his positive play. Making Jermaine O'Neal a non-factor (or worse depending on the number of touches/shot attempts).
-------------------------------------------------
Controlling and limiting Jermaine O'Neal's shot-creation opportunities is the only way to stop him from being an offensive liability.
Given the Celtics lack of alternative post presences + their desire for a post presence + the unselfishness and team orientated offensive play of the Celtics + O'Neal's mental makeup (his aggressiveness, self-belief) ... I have little to no confidence in that happening (limiting his shot-creation enough to stop him from being an offensive liability).
--------------------------------------------
If Jermaine O'Neal simply accepted his weaknesses and became a complementary player who doesn't try to create his own offense he could still be a valuable role player in this league.
Unfortunately, he hasn't been able to accept that yet.
Perhaps (and this is very much a skeptical perhaps), if we define post play as simply passing the ball into the post and letting the post player make a move AND we took into account (as you mentioned) that Perk didn't take many bad shots because he wasn't given the opportunity, then maybe (and again, I'm still not buying it) you could have an argument. However, I think you're forgetting a number of things:
1) Jermaine O'Neal was asked to be the 2nd best player on the Heat. Thus, he was expected to take a lot of shots.
2) While Doc will probably look to utilize whatever post game he has (because the C's are pretty desperate for one), simple basketball logistics state that even if Doc tries to utilize it more, he'll still get fewer shots than last year. There's no way he can put up as many shots as he did last year when he's going to have to share the floor with 4 double digit scorers.
3) I think you're letting our first round playoffs series influence your opinion about him too much. I'm guessing that outside of that series and the games Boston played the Heat, you didn't see too many Heat games. I'm thinking if we took Jermaine off that squad, inserted KG, and then made KG matchup with a defense as good as the Celtics upfront, with the teammates he had on the Heat, he probably would've had a pretty crappy series too. (Let's also not forget that all of our superstars had some pretty crappy, and even crappy stretches of games, in the playoffs).
4) Let's not forget what a dramatic difference O'Neal can make in other areas. While he doesn't have three point range, he can spread the floor much better than Perk. While he may have lost some athleticism, when Rondo or PP finds him under the basket, he will finish.
I really expect Jermaine to find himself again here. He did try to do too much at times last year. I just don't see how that's going to happen here. Stephon Marbury, despite sucking, bought into the team concept here. I don't understand why you think that Jermaine won't. Quite frankly, prior to this year, there really was no reason for him to shoot less since he was playing on mediocre teams that were expecting him to score a lot.