After seeing the technically polished escapes Grim used against Fedor tonite, I'm 100% sure that Rogers would destroy Lesnar.
Lesnar takes too many shots, even from guys with reaches much shorter than him. Grim's reach is the same, and he has 10x more power in both hands. If he can get up from Fedor (and if Herring can get up from Lesnar), then Lesnar has minimal chance.
Welcome to an era where not just one, but a few dark shadows cast over the UFC's HW division.