Total team let down. I don't think any one person is responsible for this season. Ainge didn't have a good off-season. Brad is having the 2nd worse season of his career. The players, with the exceptions of Brown, Tatum, Timelord and Pritchard have been inconsistent to poor.
Then add in the raft of injuries, games missed due to Covid, the condensed season, the fast turnaround to start the season(that I think really hurt the Celtics more than people think), the lack of Summer League for the young guys, the lack of length of training camp and the preseason, the practice facility shut down for a while, no practices and the on road protocol for Covid that basically keeps them in their hotel rooms when not at the arena and I think you have the perfect storm for the type of season they are having.
Given all the circumstances, I am surprised the team has been able to drag themselves to a .500 or so record. This team should probably be somewhere around 7-8 below .500 if you take everything into consideration.
But, I am a glass half full type of fan, so maybe I am being to lenient in my view of the situation.