Nani is probably the best winger Man U has for Van Gaal's tastes (Mata is a #10). This is a huge problem for sure. Even if Nani doesn't start every match, he will be preferred by a huge margin over Young and Valencia.
But then again, Van Gaal could have access to some serious cash to retool the whole team, in which case Nani could be available if Van Gaal buys 2 or 3 new wingers.