Manchester United are keeping tabs on Spain under-21 left-back Aaron Martin, sources have told ESPN FC.
United scouts watched Martin, 20, play for Espanyol against Levante on Friday night and the club have asked to be kept informed of any developments regarding his future.
Martin signed a new long-term contract in January that could keep him at Espanyol until 2022.
However, sources have told ESPN FC there is a growing acceptance among officials at the La Liga side that they may not be able to keep the defender beyond the summer, or even the January transfer window.
Manchester City are also reported to be interested in Martin and it is likely Espanyol will demand his full €40 million buyout clause before letting him leave.
Luke Shaw, Daley Blind, Marcos Rojo and Ashley Young have played in the position while Matteo Darmian, signed as a right-back by Louis van Gaal, started the 0-0 draw with Liverpool at Anfield on Saturday.
Shaw was named the Premier League's best left-back at Southampton at the end of the 2013-14 season before joining United in a £30m deal, but a combination of injury and loss of form has seen the 22-year-old slip down the pecking order under Mourinho.
He has made just one appearance this season as a substitute in the 4-1 win over Championship side Burton Albion in the Carabao Cup.
He played 90 minutes for the reserves in a 4-2 defeat to West Ham on Sunday but was included in Mourinho's travelling party for the Champions League trip to Benfica this week when the squad departed on Monday.
Eric Bailly is set to miss the game as he continues his recovery from the groin injury that ruled him out at the weekend.
Meanwhile, United have offered a trial to 14-year-old midfielder Rines Arifi.
The Swede, who plays for Husie IF, is set to spend a week training at Carrington later this month.