If you don't see how 14 injuries can be massively disruptive, there's not much more to be said
IMO that's an excuse. We are playing a team whose wage budget might be less than Rooney's.
We had players like Smalling, Blind, Carrick, Herrera, Mata, Lingard, Martial who were first choice players. If Van Gaal can't beat some Danish team without Rooney and De Gea then he needs to go.
Also performance wasn't any better when we had all players available.