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...