I think McTominay and Fred make a good combination, just not of the required quality. Trying to fit in both Fernandes and Pogba would require a complete restructure of the midfield set-up.We need to replace both of them in fairness as they are both holding each other back and are not a good combination in a midfield 2.
The hope should be to sign a top DM who would allow us to get Pogba and Fernandes into the same midfield most games and have Fred and McTominay as rotation options to come in and play alongside a proper DM. They are both good but limited players who can be very effective when used correctly, United just haven't been doing it.