If I am not mistaken we were up when Sullinger was subbed in. To me he looks always late getting over, but I respect your opinion. T.P.
We were up 2 when he was subbed in the 4th, he was removed when we down 6. What Sullinger was guilty of was respecting Horford's 3 too much. I wager that was more of a problem with Stevens' strategy of protecting the 3-point (which was our focus the whole game) than something Sullinger himself did wrong. Horford is not a good corner shooter, though some respect has to be given to his 3s... but not that much.
But the biggest problem we had was that we went small. With Horford removing our big from the equation, we were left with small players protecting the paint, and the pick-and-roll destroyed us. You can't leave Jerebko/Crowder to handle Millsap like that in those circumstances, with Turner rotating over. That's a recipe for disaster. And after it didn't worked early in the 4th, Stevens should've gone with a 2-big unit like he had done through much of the game to some degree of success. Even the Lee/Sullinger pair was effective.
What I took issue with was your characterizatoin of Sullinger being yanked. He was replaced with Oly on the offensive end when it was almost over and Stevens does his usual perimeter offense. Then he played the defensive replacement with Amir who hadn't played anything really in the 2nd half... some fresh legs, and regardless the unit Sullinger was part of wasn't working anyways, so no reason to bring him back in under those circumstances. He did his 5 minute stint which is normal for him.
The defensive problems were first and foremost because of Stevens sub strategy of going small and then I'm assuming his defensive scheme was exposed with Thomas defending the point and Millsap involved in the pick-and-roll with Crowder/Jerebko defending. A tall task, and Stevens never adjusted in the whole 4th.