#4 Eric Bailly
Mourinho's first signing as Manchester United manager is still one of his best. The Ivory Coast defender has become a staple of Jose Mourinho's side at Old Trafford, and at just 24 years old, he still has plenty to offer the club in the future.
Since Ferguson left, and the club lost both Nemanja Vidic and Rio Ferdinand, and United have struggled for good centre-backs. While Bailly's first two seasons at United have seen him suffer from some injury issues, he has been a rock for the club whenever he has been on the pitch.
Mourinho's biggest headache since becoming United manager has been finding a consistent partner for Bailly, and if he can do that, then United might finally be able to make a return to the glory days.