There is nothing the broncos can do to fix this in the current yr. They will have to go at it with Ball, Hillman and Moreno. You just can't find good players available week 10 of an NFL season. We got lucky on the kick returner but i doubt we find a starting RB.
This isn't a huge blow because Willis doesn't make big plays anymore. It's easy to figure RB will be a high priority next yr. For McGahee, at his age, I'd say he will be back but can no longer be counted on as a starter.