This is Van Gaal who is known for bringing up players, he made Thomas Müller into a world class player and made the likes of Toni Kroos. He's now potentially doing the same with Rashford...
If Pereira was good enough he'd play it's simple as that. He plays Jesse Lingard ffs.
Also Mata can fit the system if he has wingers besides him. Ozil has the same issues as Mata anyway and Mata at Chelsea at no.10 was fantastic. He was never the right fit for United for reasons I have stated already(Rooney and lack of top quality pacey wingers to play in tight spaces) but give him that and he's a quality player.