To his credit, he is a good back. The numbers have fallen off because the rest of the offense is garbage. No one has to respect the passing game because Losman is too inconsistent. He has the ability to make big plays with his legs or his arm, but decision-making ability is not there. Additionally, the Oline isn't disciplined. McGahee has had many long runs get called back by holding calls.
I will admit, he has had a few fumbles the last few weeks, and he is certainly not the best running back in the league (I could see a claim to be top 10, though), but it can't be coincidence that he was underwhelming until Holcombe became the starter, had good games this Holcombe under center, then had poor games once Losman got back in the game. Holcombe has the ability to make just enough plays to put the other offensive players in good position to perform.
Not looking to ruffle any feathers - I am just as upset with his performance the past few weeks since he guaranteed I would be out of the playoffs - but I am starting to think that talent has a lot less to do with fantasy success than the player's circumstances do.