Firstly, with the exception of last night, Horford has had a very good series. He was arguably the best player in Game 1 (24 and 12 on 17 shots.) Game 2 he had a solid night as well (17 points on 6-11 shooting). Game 3 he didnt score much, only 8 points, but added 13 boards and 6 assists. Last night he was a bit neutralized, although part of that is because Millsap was going off -- the proper offense for the Hawks was to force it to him. I think his defense has been very good.
The Celtics had to go small because Horford, not Millsap, was killing our bigs in the first two games. I don't know if it's on the Celtics ball denial, Atlanta's coaching, or Horford himself, for not posting up to take advantage of that. Probably a bit of all three, but I'd give the Atlanta coaches most of the blame.
Anyway, if the Celtics win this series, I think it would help their case. Two months later he'll remember how tough the Celtics played, even with one of their best players out. And he'll know that the Celtics backcourt is so much better than the Hawks. As a big, you want to play with a great backcourt. That does not describe the Hawks. And Horford knows it.