I guess i have different opinion what backup player is in a big club which chase trophies. Many United fans think that player who is not good for first 11 is good for backup. I disagree.
You can't have 20 world class players of course but club like United can and should have 20 players who are good enough for first 11.
Season has 60 games. You will rotate a lot and you need quality. What is use of an average player who is just a body on the pitch and with who performances of the team drop significantly. So imo, players like James, Lingard, McTominay, Jones, Rojo, Pereira....are not good enough for backups. Fred, Matic, Bailly, Romero are good backups.