Here is a quick list of centers I'd pursue mid-season:
Horford
Hibbert (buy low?)
Lopez (buy low?)
I'd certainly be interested in Hibbert if we could get him. He's an absolutely elite shot blocker, a solid (if not elite) rebounder, and he can also contribute some on offense.
I wouldn't touch Lopez. He is walking scarily close to the Greg Oden / Andrew Bynum path with his seemingly never-ending injuries. I went through enough heartbreak with injuries during Boston's "Big 3" run, and I don't want to go through that again. To make matters worse Lopez (at 7'0" tall and with a huge wingspan) rebounds like a small forward and defends like a 35 year old. He really is a one-dimensional player who can only really help a team with his scoring.
Hortford I love as a player, but the health side is a bit scary for him as well. He's missed a LOT of games...though he's probably a safer bet then Lopez.
Another guy I'd look at is one of the Detroit due - either Josh Smith or Greg Monroe. I suspect one of those guys will be one by the trade deadline, and Boston might be in a good position to snatch one of them on the (relative) cheap.
Josh Smith is not a centre, but he is extremely mobile/athletic, an above average rebounder, a skilled inside scorer, an excellent rim protector, and one of the best passing bigs in the NBA. If you stick him in his natural PF spot and ban him from shooting threes he could actually be the perfect complement for someone like Kelly Olynyk. He is quick enough to switch on to KO's defender (if he gets beat) and he can also protect the paint against penetration - the two areas where KO is weak. KO on the other hand is an excellent outside shooter and can stretch the floor - the one area where Josh Smith is weak. They would also make one of the best passing frontcourts in the league.
Josh Smith would also fit in very nicely with the 'run and gun' and 'move the ball' style of play that Stevens is trying to implement thanks to his deadly transition game and his (aforementioned) passing ability.
If they decide to keep Smith and trade Monroe, that works too. He's a big with legit centre size 6'11", long wingspan, solid frame) who can score efficiently in the paint, rebounds at an excellent rate, and can step out and hit the midrange jumper - plus defensively he is a definite upgrade over Sully.