Pretty big because they run A LOT behind him. They run off tackle to the right so much. All you have to do is go back and chart every play against Denver last season. I did it when I was breaking down Nolan's defense and it was significant. If you look at Mendenhall's rushing splits, he ran right 18 more times than up the middle (and 22 more times than left) and averaged 5.3 YPC.
Well, if they developed Garner the way I see it they can develop someone else as well and Gardner and Murtha have better physical tools than Garner. So if you develop either one of them, you have a better player that they spent the time developing.