Knile Davis, Michael Dyer, Murrayand Lattimore should all be in the conversation as well.
The reason Relf might be the guy is due to the dropoff from him to Favre. Now Im not saying Favre is not going to be a good QB, but Relf fits Mullen's offense perfectly and he gets first downs on his own all the time. Going from a 6'3" 235 lb QB to a 5'10" 180 lb QB is a big change, and I think you would have to change the offense a bit. I think with Relf MSU can win anywhere from 6-9 games this season. Going from Relf to Favre I think could put MSU's ceiling at 7 games, that is pretty big drop.
With Davis Arkansas still has monster WR's and Wingo and Green backing up, so even though there is a dropoff its not the same as above.
I think Dyer is probably third. The drop off from him to McCaleb is pretty significant. Im not sure how many games losing him puts in play. I think AU win Dyer maxes out at 8 wins, so without him they probably drop to 7 max.
Lattimore might be the top guy. He was dominant last season and the dropoff from him to Kenny MIles (Had to look it up) is pretty significant. South Carolina is in a wide open East and without Lattimore Im not sure they can beat UGA or FL.
As for Murray, I think its the same situation as Relf. He fits Richt's pro style offense perfectly and the drop off from him to Hutson Mason (WhoI think MSU almost signed) is significant. I think without Murray they may not be able to win 8 games. With Murray, 10 games is certainly possible due to the most favorable schedule I have seen in a long time.