Both Manchester United and City need wingers, and Mitoma will be fit in both clubs, just that the player will be the one to decide which club he will prefer to join in the January transfer window, that is, if Brighton will be willing to let him leave in the transfer window.
Rather than looking for a new winger, it's better for Manchester United to sign a new goalkeeper and few defenders. Their midfielder and striker already strong enough, so they just wasting their money to sign a new winger.
I am not denying it, but if their injured defenders were all fit, their defence would be okay to go if they had good wingers in the front that could create and convert goal chances. Keeper is another problem they are facing now because they sold their best keeper during the summer and signed Andre Onana from Inter Milan, but unfortunately for them, Onana is not performing to what fans expected, and he is causing some errors that are leading the team toward losing matches.
They can be managing Onana for now and be given their second-choice keeper some chances in order to provide competition for Onana.
A player will join a big and tough team, Manchester United only a big team, so Mitoma should be in Manchester City.
Manchester United is also a big and tough team; they are just facing some setbacks that will soon be over, and they will regain their balance and continue with their good performance. If Mitoma joins, then it can be another opportunity for them to be tougher than before.