Well, in my opinion, Glenn wont be a bust. He has the speed, the size and the heart to be a star again in GB. And he has the QB and the system there. I think he will do great.
But if he really busts, I am not sure Ferguson will break out then. The Pack is relatively thin on proven WRs, and if Glenn fails, there is no guarantee that one (probably Donald Driver) will emerge. But that would mean, Ferguson will have to carry that load himself alone, and I am not sure if he is a to double-coverage after double-coverage.
But again, they have a QB who makes his WRs better, and so I think both Glenn and Ferguson will have solid but maybe not monster seasons!