Fatigued might be one thing, but I think he was also lacking focus and proper motivation after the turmoil to the poor start of the season.
He was way better once he returned to injury, not at the level, but a step in the right direction from what we saw from him defensively in his rookie season before needing his back surgery.
Some of it might be his being overweight, but he shown it throughout his career. The guy is not a race horse, he is a draft horse. He does some things well but will never be fast.
Lack of focus and a motivation in a pro, is on the pro when you have a coach like CBS. It is possible that he was mad at Ainge for not getting him his rim protector. Ainge promised him one and we got Zeller, whom I like as a decent backup, but he is hardly a rim protector. For those who do not recall.
http://www.celticsblog.com/2014/5/10/5705862/jared-sullingers-father-satch-hints-that-the-boston-celtics-are-acquire-rim-protector-rockets-asik
I don't imagine Ainge liked reading that either.
"Whole career" which includes 1) His rookie season, in which he actually played good defense for us, 2) A second year in which he was recovering from his back surgery, and 3) Last year which there was no excuse for, but hopefully fixable explanations.
I'm counting on a return to point 1), and we saw some of that once he returned from his stress fracture and more importantly during the playoffs, even you have admitted seeing some of the same in the past.
Lack of motivation is a problem, but this is also a young kid and it's something that is addressable... I think the culture of losing we had built since his return got to him a bit, a self fulfilling prophesy of course once you fall into the vicious cycle that gets created while losing.
You also keep pointing at our improvement of play once Sullinger went down, a notion I debunked quite a few times previously. Our team had actually begun to play better BEFORE Sullinger went down, and that occurred just prior to the influx of talent starting with Isaiah Thomas. But of course, we should ignore that stretch because it doesn't jive with the narrative of "Sullinger the useless player".