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Manchester United have hardened their interest in Celtic defender Kieran Tierney as the English Premier League club prepares its summer transfer strategy.

With Jose Mourinho demanding an overhaul of his defence, Tierney's name is being pushed by United's scouting department.

The Scotland star signed a new six-year contract with Celtic last October and the champions are likely to want upwards of £20m for the player who has impressed United during a season in which he has captained both club and country.

The 20-year-old's ability to operate in both full back positions and as a central defender is of value to Mourinho, who prefers footballers capable of working in a range of systems.

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United need a starting left back after a season in which the position has been shared between converted winger Ashley Young and the chronically underperforming Luke Shaw, whose advisors are now actively looking for new employers.

Mourinho's first choice for the position is Alex Sandro, but Juventus have been asking a transfer fee of over €70million for the Brazil international.

Tierney has been shortlisted by United's scouts as a less expensive option, and one who has also demonstrated an ability to play on the left side of a three centre back system this season.

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While Mourinho's preference is to defend with a back four, the Portuguese has started with three central defenders eight times in the current campaign.

Mourinho has made it clear that his squad requires significant reinforcement in both defence and midfield to compete with a Manchester City team that has benefited from substantially greater investment in transfer fees since he moved to Old Trafford.

United's board is understood to have agreed to move on underutilised defenders such as Shaw, Matteo Darmian and Daley Blind, who have failed to establish themselves as regular starters.

Tierney has been monitored by United since breaking into Celtic's first team as a teenager.

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Reservations over the Scot's readiness for the Old Trafford squad have diminished as he's proved a reliable figure at both Champions League and national team level, including a well-received performance at centre back in November's friendly international against the Netherlands.

A summer move for Tierney would depend on persuading the defender to leave the club he supports.

Celtic will attempt to market Moussa Dembele to the highest bidder again in the summer, and achieving a high transfer fee for the France under-21 striker would make the club more resistant to offers for Tierney.