The most obvious issue for United is the problem created by Bruno/Pogba/LW. You can only have two in the team at once but they're all star players, so someone has to miss out.
If you go with Fernandes in front of McTominay and Fred, it means it's either Pogba or Sancho/Rashford on the left. If you put Pogba alongside McTominay and Fred, you then either push Fernandes wide or drop him.
United have created a problem similar to what Arsenal created when they signed Ozil. They were a better team without him but if you try to shoehorn too many players into a system, it just doesn't click. The following midfields are better in my opinion:
McTominay - Fred
Greenwood - Fernandes - Pogba
Or...
McTominay - Fred
Greenwood - Pogba - Sancho/Rashford