If you make the Bulls game a "normal" blowout, say 30 points, and get rid of the extra 26 points, the Celtics still have the 3rd best net rating in the NBA, by about half a point. Of course, then you have to adjust the 4th place Nuggets' 45 point win against Atlanta, and so on.Out of curiosity, I looked a bit into the Celtics results this season.
They are 16-14 in games decided by 10 points or fewer. Those include some narrow victories over teams they really could have done better against (Atlanta and New York) and a loss against Charlotte and Phoenix. Not sure how to categorize the loss to Brooklyn. By record, they shouldn't have lost that game, but BKN has been on an absolute tear.
They haven't been blown out this season, with only 5 losses by more than 10 points. Their worst loss coming against Miami - which is a fairly middle of the road team, interestingly.
On the positive side, their point differential is significantly impacted by the 56 point drubbing they gave Chicago. That's a huge point differential, and really helps their average numbers. They have had 8 other blowout wins (>20 points)
This is a very good team that can get better (mainly by shortening rotations). They definitely have chemistry issues though, and also effort issues against mediocre/bad opponents. If they locked in against bad teams the way Milwaukee does, they'd probably be 2nd in the East, with about a +8 net rating.