I want to see Michael Keane and Blackett against Madrid. Smalling's injury means that we'll probably see both of them, which is great news. If we don't sign any new defenders, the only positive outcome of it would be Blackett and M.Keane getting loads of games in the 3-5-2 formation.
Van Gaal already said that he likes both of them, so maybe they've shown enough to convince him that they have the right mentality and enough intelligence for their young age to actually make their mark in the first team this season.
I still think that we need at least one more center back though. We're obviously going to stick with this 3-5-2 formation, so we probably need more than 5 center backs when 3 of them will be starting games.
No matter what the next month brings (or doesn't) on the transfer front, I trust Van Gaal completely to guide us to a successful season. Exciting times ahead.