Having signed a defender, a midfielder and a centre-forward, Mourinho is now on the lookout for one more reinforcement, preferably a winger who can cut inside and play with the midfielders and attacking players.
Sportsmail examined United's potential targets, one of which will likely make the switch to Old Trafford this summer.
Ivan Perisic
Strength and pace down the wing, the 28-year-old is reportedly keen on the move, but Inter are holding out for their £48.5m valuation to be met.
Christian Pulisic
Dortmund are determined to keep the skilful 18-year-old, also targeted by Liverpool. United will have to move quickly and splash out for the future star.
Arjen Robben
With one year left on his contract at Bayern, the 33-year-old may be replaced by younger players. Mourinho could favour a reunion with the former Chelsea man.
Gareth Bale
The Welshman's future is still uncertain after a season overshadowed by injuries and criticism. If Real Madrid decide on selling the player they will want to recoup most of the fee they originally paid in 2013.