I see Culbert remaining at running back for this season, for two reasons:
1. It's difficult in this day and age to get by with just two running backs. The players are so big and so strong, the hitting so fierce, that you have to anticipate that you'll need at least three backs ready to play. Even if Lucky and Glenn don't get injured, it's still a long season and the wear and tear can limit your effectiveness.
2. Culbert is the only candidate for the backfield - other than Glenn and Lucky, of course - that even has a hint of how to pick up a blitz, when he will be the "hot read", or a grasp of the terminology. While there are exceptions to every rule, I find it bordering on impossible for a freshman back to come in and be an every down player and be able to make all of the assignments that are required under the West Coast offense. The only way it happens is if there are no other backs available.