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 think they already had Onana and no matter what criticism will have towards Onana, they are gonna stay with him for quite a while I feel. I do think they need a good winger on the right hand side. Antony is not that good in my opinion.
A player will join a big and tough team, Manchester United only a big team, so Mitoma should be in Manchester City.
Mitoma is a good choice both City and United. Mitoma is a very good winger. With City they have already got wingers but since they like to have density and Guardiola also praises the player, it's possible that City might make a move in future.