The Celtics do not necessarily need a "rim protector," per se, as I think that term is too depreciative. They simply need a defensive-minded big. Think about it: Sullinger, Olynyk, and Zeller are all offense-first big guys, and the Celtics still had an average to above-average defense. That's amazing! A defensive-minded big would do wonders for the team, no matter what their "speciality."
What the "modern" defenses now do are: 1) channel people to the sidelines and 2) switch constantly. This scheme makes the ability to switch and move laterally more imperative than the ability to block shots. What defenses used to do is force guys towards the middle of the floor into a shot-blocking big(s), but the advent of soft, wiry seven-footers shooting threes has made that strategy hard to execute.
However, a defensive leader on the floor (like KG or Bogut) who can see everything initially and quarterback his players helps put teams over the top. What you see with the Hawks and Warriors is predicated on constant switching and communication. It is very much a team initiative. So I honestly think that, yes, a big man is incredibly important to help put a defense over the top, but length on the team overall can be just as big a factor. That's why I think what the Bucks are doing is interesting. They can eventually get the frenetic, constant-switching defense like the Warriors, but instead of some possessions ending in 6'3" Stephen Curry being posted up, the Bucks' defensive possessions will end in 6'7" MCW or 6'11" Greek Freak being posted up.
If the Celtics get a big that can move their feet (like KG) and alter shots, they no longer have to "ice" every screen, and the guards will be in much better shape. However, with that comes the need for overall length and defensive proficiency. Likewise, if they get an intimidating "shot blocker" who is more apt at roaming the paint and calling out screens, that could put the team over the top also. I have no doubt that Stevens would come up with a grat scheme based around our personnel, but it is not the only option, and it's on the entire team (current bigs included) if they want to get their foot into the door of being an elite defense. They were already pretty decent (especially considering our bigs), but I think a defensive big man, no matter what his speciality, with length is what the team needs, not necessarily a "shot-blocker."