The Celtics were 8-8 between Dec 16 and Jan 19 (excluding this last GSW game). 8-8 sounds pretty mediocre but they were +6.8 in those 16 games which is a very strong point differential.
They did not look good in that stretch. It was frustrating because you know they can play better. I think they are coming out of it now. This last game vs. GSW was a big win, but the Celtics were still not playing at a championship level. Consistency is still an issue It appears from the stats from previous defending champs that it is pretty normal.
No need to fire the coach or blow up the roster. But we have some work to do.
The frustrating thing was the inconsistency and the fact that they lost to teams you would back them to beat 9 times out of 10, like Chicago, Sacramento, injury ravaged Philly, Toronto, Atlanta. Here are those losses:

6 of the 8 losses were because we shot below 30% from 3, 3 were by 4 points or less...the only one where we really got blown out was the Toronto game and we beat them by 50 pts earlier in the season. When I look at those things I'm not thinking our team is technically flawed or we need a piece or there's some structural issue, I'm thinking lack of concentration, leading to lapses, leading to lazy shots that are missed, leading to losses. That's what Joe has to address, the concentration and those types of lapses, like what happened in the Atlanta game that we gave away, or any of those other games we lost by close margins, and another few where we could have lost as well.