Jose Mourinho is desperate to add to his squad as he looks to close the gap on table-topping rivals Manchester City.
Veteran winger Ashley Young has performed well in the unfamiliar role so far this season, while outcast Luke Shaw has started the last three Premier League matches.
But Mourinho has demanded a world-class left-back and United have been heavily linked with a swoop for Tottenham star Danny Rose.
According to The Daily Record , the club’s number one target remains Juventus defender Alex Sandro.
However the Serie A giants are reluctant to let the Brazilian go and insist it would take an offer of €70m (£62m) to tempt them to sell.
United are looking at alternatives and scouts have identified Celtic’s Kieran Tierney and Fulham’s Ryan Sessegnon as younger, more economical options.
Tierney, 20, already has Champions League experience, while 17-year-old Sessegnon is one of the most coveted youngsters in the country, with Spurs and PSG also believed to be vying for his signature.
Left-back has been a problem position for Mourinho since he took charge last summer, with several players already tested there, including Daley Blind, Matteo Darmian and Marcos Rojo.
And while the former Chelsea boss has been impressed with Shaw’s resurgence, doubts still remain about his fitness and consistency.
United are currently second in the Premier League, 15 points behind fierce rivals City.
They return to action this evening as they host Championship side Derby in the FA Cup.