The Real Madrid star has just one year left on his current contract, making him an attractive target for the club.
United believe he’s the ideal person to partner Harry Maguire at the back, while also considering him to be an upgrade on the likes of Axel Tuanzebe, Victor Lindelof and Eric Bailly.
At right-back, they’re big admirers of Kieran Trippier.
They’ve already had one bid rejected for the Atletico Madrid star but could return with another by the time the transfer window slams shut.
And when it comes to bolstering their midfield, Rennes youngster Eduardo Camavinga is their main aim.