#3 Victor Lindelof
Okay, maybe we are stretching the youngster thing a bit too much, but the Swede is only 24 years old and may be considered a young man at the least.
He is the same age as Bailly, who is already United's best defender and hence is not a part of this list. But Lindelof should be the ideal partner for Bailly this season and his performance in the World Cup alone is enough to justify that.
Lindelof looked assured and confident and that is what a little trust from the people around you can do. Last season, Mourinho shuffled around his defense a bit too much for anyone's liking and coming into a new team, in a league as competitive as the Premier League took its toll on Lindelof.
Yet his attributes make him the perfect partner for Bailly. Bailly is strong and fast, but not so good with the ball going forward. Lindelof is very good with the ball and can initiate attacks from the back.
Bailly can be a little eccentric at times, but Lindelof is a picture of calm. Together, these two can form another formidable defensive partnership that has been the mainstay of United's title-winning sides of the past.
As such, Lindelof needs his manager to root for him, to guide him and yes, again, to give him a little trust. The Swede definitely deserves that.
Mourinho needs to stick to a formation and use Bailly and Lindelof continuously so that these two get a run of games together and builds an understanding between them.
Also, rendering a person useless at the slightest hint of an error is a big no-no. Everyone needs time, and support and if Mourinho could man manage the Swede, Lindelof could be the answer to United's defensive worries.