#3 United's right-wing issues
Jesse Lingard, Juan Mata, Marcus Rashford, Anthony Martial and Alexis Sanchez have all been deployed on the right wing but none of them have lived up to the billing.
United's right flank is perhaps the weakest link in their system and Mourinho simply must fix the issue if they are to challenge on all fronts next season.
Neither of the above-mentioned players likes to hug the right-touchline and receive the ball in the wide areas. All of them like to cut inside and play at close quarters which is asking too much of Antonio Valencia who has to maraud up and down the right flank.
In Willian, you have a player who can do both and someone who, over the years, has mastered the act of finding the balance between the two - staying wide when required and cutting in when the situation demands.
His high work rate is another major plus and for Mourinho, who adores players with a great work ethic, he can directly slot in at right-wing and make United more dangerous.