The other good thing about this situation is that, at least in Indy, Manning was always one of the better quarterback at recognizing when the defense was defending the pass too much, and would frequently call an audible to pick up an easy 4-5 yards with a quick run. If Addai can do it, McGahee can do it.
While McGahee is going to lose the benefit of a mobile Tebow, he's going to gain the defense's respect for Manning and thus never more than 7 men in the box, and he's going to be running in more ideal situations against a nickel defense.
I was very high on him in July, started to wan a bit lately with some Hillman hype (mostly due to Lammey's reports), and now am back on the bandwagon. Should at least start the season strong in the first 4-5 weeks.