I think we'd better wait and see how Celtics perform on defense with Shaq and JO and without Perkins. IMO, Perkins defense is somehow overrated. The truth is Shaq is also one of the best post defender, just like Perkins. Shaq's only weakness is his pick and roll defense.
Considering Shaq's offense is far better than Perkins. So overall, Shaq provides much more positives than negatives.