With Graham, it's not the other RBs on the roster I'm concerned with at all, for the same reasons already stated by LS2. It's right guard Davin Joseph's broken foot I'm a little concerned about.
I think this is what you're seeing in the recent pre-season stats; not so much Graham's drop off in talent.
I believe the rookie Zuttah is stepping in for now. He's pretty good from what I hear. Huge student of the game. Played all along the O-line in college and knows how it all plays in motion as a unit. But still, he's a rook, so that could go either way. I'm sure mistakes will be made here and there and Graham will occasionally have to pay for them, but I don't think it means the end of Graham.
I can see maybe his production being a little sporadic in the first few weeks of the season, but barring any set backs, Joseph should be back in time for those cold/bad weather games when RBs tend to get a little more work in.