Manchester United are lining up a summer move for Celtic defender Kieran Tierney, sources close to the Scottish club have told ESPN FC.

United boss Jose Mourinho has made a new left-back one of his priorities this summer and Tierney has emerged as United's top target.

United have been linked with Tottenham's Danny Rose in the past, but they now see Tierney as someone who can be a long-term success at Old Trafford.

Tierney is regarded as one of the best prospects to emerge from Scotland in recent years and has already captained Scotland despite being only 20.

Left-back has been a problematic position for United for some time with Luke Shaw failing to convince Mourinho and Ashley Young being converted into a makeshift full-back.

Celtic's Kieran Tierney could be the answer to Manchester United's left-back problems. Getty

United have scouted Tierney on numerous occasions during the last 12 months and they are expected to watch him again in Sunday's Old Firm Scottish Cup semifinal against Rangers.

Sources have told ESPN FC that United have sounded out Celtic about their interest in Tierney and they are confident of agreeing a deal for the defender.

Celtic are reluctant to lose one of their prize assets, but accept they cannot compete with the riches on offer from the English Premier League.