Basically, I think we're not so hard up that we absolutely need a 7-footer at the 5. Or, put another way, with JO, KG, and BBD as 3/4 of a nice big man rotation, the height of the 4th guy isn't that big a deal. I think there is sufficient versatility and workable combinations among those 3, any of which can line up at the 5, to make working out matchups rather simple, even if the next guy is an athletic PF who isn't 7 feet.
I wouldn't say we absolutely need a 7-foot center, either. I think what we do need, however, is a back-up center who can play adequate defense against a 7-footer with a decent jump shot. It's just that those guys are more likely to be around 7 feet tall rather than your 6'9" bruiser who is better suited to power forward but can play center if needed.
Let's look at this from another angle. Assume KG and JO are on the bench. Maybe it's due to foul trouble or injuries (and both should be considered injury risks). What qualities should the Celtics be looking for in a big man playing in a line-up next to Glen Davis? I might be inclined to want a center with the ability to block some shots and make put-backs without getting blocked.