The Red Devils are working hard to secure a move for the Juventus talisman and are confident a deal will be pushed over the line.
The club are holding back on confirming the squad numbers for the upcoming season as they continue to negotiate a deal for the player who left them on a free in 2012.
A United source told The Sun: “It would be very expensive for the club to release the squad numbers only for them to change.
“The club want to make sure all players are signed and sealed ahead of the coming season before they are announced to the world.”
The Frenchman wants to bring in a versatile defender that can play across the back-four and he believes he can tempt both youngsters into moving to the Emirates.
The Sun claim the pair would cost between £10m and £12m each.
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Reports in Spain suggested the striker had handed in a transfer request.
But the 27-year-old is in Antonio Conte’s plans next term and the Evening Standard claim Costa is in training with the rest of the squad at their pre-season training camp in Austria.