After reviewing several of his games from 2012, it is my contention that he was removed from the Nickel because he got fatigued playing 100% of the defensive snaps. He was visibly fatigued in the last game where he played in the Nickel; although, that game was in Denver which obviously contributed to the problem.
I think he has the talent to play Nickel if his weight is down at the lower end of his weight range; however, his real value is as the MLB in the base defense. Even if his weight is down, he's a really big guy to be running around back there for 100% of the snaps.
I expect to see Holloman as one of the Nickel LBs. I guess that Carter is most likely to be the other one, but I really don't know. If a 3rd Safety steps up his game in TC, then they could move Church up to play LB in the Nickel in some situations with the 3rd guy replacing him at Safety.