There are no perfect comps but Clint Capela comes to mind. People will say "oh but the passing" but he uses it so infrequently and I doubt any team is going to use his play making skills anymore than the C' do.
Besides that, the history of the NBA pretty much says players are what they are at 25. He's 25 in October. I don't see much room for growth.
I think Capela is a superficial and lazy comp for Rob.
First of all, Capela isn't in Rob's league as a shot blocker. For his career, he averages 2 blocks per 36 with a block rate of 4.8%. If you just look at him through age 24 (to maych him with Williams) those numbers are about the same. Rob is at 3.1 per 36 with a block rate of 8.0%. Rob's worst block rate, 6.8% this year, is better than any single Capela season outside of his rookie year when he played 90 minutes.
Capela is a bit better of a rebounder, mostly due to what he does on the defensive end.
Rob is also better at steals. Capela has 0.9 steals per 36 for his career with a steal rate of 1.3%; Rob is at 1.4 per 36 and 1.8%.
And Rob is far better at passing the ball. Capela averages 1.3 assists per 36, assist rate of 6.2%; Rob is at 2.5 per 36 (2.7 since his rookie year) with an assist rate of 10.5%. He has about one-quarter of the NBA playing time that Capela has, but more than half the assists. In 471 NBA games (410 starts), Capela has a career high of 6 assists, which he did once. Once in 471. Rob has done 6 assists or more 4 times in 174 NBA games (77 starts). And it isn't just that he gets assist but the type of assists. In his whole career, Capela has generated 1,164 points off of his assists, 3.3 points per 36. For Rob's career, he has generated 613 points off of his assists, or 6.4 points per 36.
Finally, Ime has wanted the Celtics to play fast, 0.5 second basketball. Get the ball and make your decision to drive, shoot, or pass immediately, within 0.5 seconds. Derek White is the Celtic who most exemplifies this, but Rob is right there behind him. The ball rarely sticks in his hands in any situation on the court.
Rob does a lot that Capela has never done and won't ever do.