I'd say FB. We need some depth there. LB and RB are both loaded, and I also think he is too big to play RB. What everyone is saying is that he will either play LB or FB.
LB. He was told that he was going to play LB when he gets back and he is already quite a bit bigger from reports. I wouldn't be surprised to see him get a chance at FB/HB/TE, though.
Donald
Walker - may be a DE though
Mayo
McCoy - Sr next year though
Davis
Thompson
Austin Johnson will be coming in
Frazier
Barnes - talk of him moving to LB as well
McCoy won't be a senior next year, Mayo will. Don't forget about Adam Meyers-White. I have been hearing a lot of good things about him. Wouldn't be surprised if he saw he field quite often this season.