The Celtics biggest weakness is big depth, not necessarily specific to center over PF but just the overall big rotation. That is true. But I don't think that is an issue. Grant Williams is as good as most back up bigs that are still playing (on a playoff team).
The Celtics have "worked around" this weakness by going with the 1 big line up, making RWill the back up center. Most on here prefer the 1 big line up so I don't understand why that would be suddenly viewed as a issue. We have 7 players who would be starters on most teams; 3 Guards, 2 Wings, and 2 Bigs. Of those 3 groups, the bigs is the weakest. A different bench center doesn't change that.
If we were using Grant more and the 1 big line up less, Al could essentially be the back up center (or RWill, however you want to look at it). But we are choosing to go small, not play Grant, so any back up center that was say as good as Grant, wouldn't play either.