The 18-year-old will wear the number 44 and is one of a few changes for the new season.
Marcos Rojo has taken Michael Carrick’s 16 while Marcus Rashford will wear No 10.
Sergio Romero has moved to 22 with Diogo Dalot taking 20, Joshua Bohui is 46, and Angel Gomes retains 47.
Jose Mourinho was frustrated by Manchester United’s transfers this summer and the failure to land a new centre-back.
The No 5 remains vacant, the number worn by Rio Ferdinand in his heyday, with Victor Lindelof remaining at 2 and Bailly at 3.
Chris Smalling wears the number 12 with Phile Jones wearing 4.
Mourinho was forced to deny he has fallen out with his players following his pre season comments this summer.
“I have my players and I like my players. I like to work with my players,” he said.
“When you repeat 1,000 times that my relationship with my players is not good, it’s a lie – and repeated 1,000 times it is still a lie.
“I like my players and my group. I enjoyed last season, the fight to finish where we finished and to manage the best position this club has had in five years. I’m going to enjoy this season.”
But Mourinho is understood to be annoyed with United chief Ed Woodward for failing to land his targets which included a new centre-back and right winger.
He said: “It is time for me to stop thinking about the market and focus on what we have.”
He will be without six players tonight for the Premier League opener against Leicester – Nemanja Matic, Ander Herrera, Antonio Valencia, Marcos Rojo, new boy Diogo Dalot and No 2 goalkeeper Sergio Romero.
And is likely to come up against Harry Maguire, who he was unable to sign.
There is also a developing situation with Paul Pogba, who is linked with a move to Barcelona, and his unrest at the club.
But Mourinho insists he is fine and happy at the club.
Mourinho added: “My perception of the situation? I see Paul very well. He arrived Monday, happy, proud, with a desire to work.
“He worked amazingly well Monday, Tuesday, Wednesday, and he’s one of the players that I have to speak to see how physically and mentally he feels because we need their help.”