Manchester United planning approach for Celtic's Kieran Tierney, left-back keen on Old Trafford transfer

Manchester United are considering a move in signing Celtic defender Kieran Tierney as Jose Mourinho prepares to strengthen his back four in the summer transfer window.

According to the Mirror, the Red Devils manager is keeping a close eye on the 19-year-old's development at Celtic Park and his side's scouts have watched him in action for the Scottish champions.

The Portuguese tactician is unhappy with Luke Shaw's progress at the club and the former Southampton man has fallen down the pecking order, behind Daley Blind, Matteo Darmian and Ashley Young.

Shaw has failed to impress Mourinho in the training sessions and was left out of the squad in United's goalless draw against West Bromwich Albion. The former Real Madrid and Chelsea manager admitted the England international is a "long way" behind the other players playing in the same position.

"It's difficult for him to be on the bench. Because I cannot compare him with Ashley Young, or with Matteo Darmian, or with Daley Blind. I cannot compare the way he trains, the way he commits, the focus, the ambition. I cannot compare. He is a long way behind," Mourinho explained, as quoted by Sky Sports.

Shaw has seven starts to his name in the 2016/17 season and last appeared in his side's 1-1 draw against AFC Bournemouth. It was his first appearance in the Premier League since the goalless draw against Burnley last October.

The Mirror claims that United are willing to allow Shaw to leave the club after the end of the season. Mourinho has identified Tierney as the player to replace the 21-year-old at the Old Trafford club and are preparing a £10m ($12.4m) move in securing his services. Celtic are unlikely to accept United's offer and the Premier League giants have to improve their offer in order to convince the defender's employer to sanction his sale, the report added.

Tierney had a breakthrough season during the 2015/16 campaign, displacing Emilio Izaguirre as the first choice left-back at the club. He was rewarded with a five-year deal last summer and his form for Celtic has attracted interest from the top clubs in the Premier League.

United's local rivals Manchester City, Premier League leaders Chelsea and Arsenal have all maintained their interest in the full-back. However, Tierney is keen on rejecting offers from other English clubs in order to complete a switch to Old Trafford.