Capable, perhaps, but definitely not good enough, at least not for our ambitions of being a title-winning side. The only reason I see for Hummels playing as a #6 would be if every midfielder, along with Rooney, was injured.
Anyway, I'm fairly sure Hummels will not be bought for that role.
I'd be all for playing him as a 6 if Van Gaal felt it provided the balance to play Herrera as the 8, Mata as the 10 and two attacking wide players flanking Rooney. With Smalling and Rojo behind him I think that would be a formidable side and as someone mention above would be a great 3-4-3/4-3-3 foil.