#4 Henrikh Mkhitaryan

Henrikh Mkhitaryan was sold by Jose Mourinho in that infamous swap deal involving Alexis Sanchez from Arsenal. Many United fans have said that they would have been better off with Mkhitaryan as Sanchez has done nothing to justify his huge wages at Old Trafford.
While Mkhitaryan also isn't tearing it up at Arsenal but the Gunners still had the better deal. The Armenian didn't really get on with his Portuguese boss and his style of play. But under Solskjaer, Mkhitaryan would have really thrived in that number 10 role.
He would have got more freedom to express himself and he may have been the creator in chief for the Red Devils.
#3 Daley Blind

Daley Blind was sold by Manchester United at the start of this season and this was another case of Jose Mourinho not backing a superb talent. Blind was Manchester United's best centre-back on offer when he was sold, and that really infuriated the United supporters.
Also, since Blind's switch to Ajax, he has been in scintillating form. There is no doubt that a player of his capability would have been backed by Solskjaer and not only that, he alongside Victor Lindelof would have made United's defence more stable.
Blind is a versatile player whose technical ability even allows him to play as a midfielder and he would have been a clear hit under the Norwegian if he wouldn't have been sold by Mourinho.