Montgomery wasn't even that good in college. It amazes me people thought he was going to suddenly become good at the NFL level. The only thing he does well is break tackles, but usually only after he picks up a head of steam, which is almost never playing behind this offensive line. He can't make something out of nothing, he has no gamebreaking speed, and he doesn't make anybody miss in the hole.
He's a complimentary backup RB at this level, and I've seen absolutely nothing in his 2 years to indicate otherwise.