I'm not sure next season Morris will be an obvious upgrade. Based on health, weight, attitude, etc it's easy to write off Sullinger. But are you certain that 23 year old Sully is going to be dramatically worse than 26 year old Morris next season? I'm not.
For me it's mostly about the fact that I have faith in Markieff's ability to play close to 82 games, average 30 minutes or more, and shoot over 45% from the field while making less than $10 million per year for the next 4 years.
I don't have much confidence that Sully will play more games, more minutes, or shoot better from the field, and I think he'll probably cost more than $10 million per year to keep unless he suffers a major injury or his weight balloons again.
Trading one power forward for an older power forward who might be worse next year?... just seems like a waste of time.
To me it's about the opportunity to get somebody who is (a) a proven starter, (b) cost-controlled and (c) under contract beyond this season, and also consolidate some of the roster excess (dump Evan Turner, basically).
That's a really good value, and none of the handful of power forwards on the Celts right now checks all of those boxes.
Again, I want to be clear that I don't think trading for Markieff would be such an amazing move or anything. I just think it's a pretty easy decision to make, if the price is reasonable.
Sully, Turner, and the T-Wolves pick is what I'd offer. If they want James Young or a Nets pick or something thrown in, forget it.