The Red Devils have been linked with a number of different full-backs ahead of the 2018 summer transfer window.
Manchester United have struggled to fill the left-back role under Jose Mourinho given his reluctance to use England starlet Luke Shaw.
The 20-time English champions are expected to sell Italy international Matteo Darmian in the summer with Juventus interested.
Manchester United could look to use Darmian as a makeweight in a potential summer deal to sign the Brazil international.
Sandro was linked with Chelsea FC last summer but Antonio Conte was unable to raid his former club for the South American.
Reporter Castles confirmed that Manchester United are still working on a summer swoop to sign the Brazilian defender.
“Jose wants two full-backs this summer,” Castles told the Transfer Talk podcast.
“The one that he wants as a proper starter is a left back because it has been a problem position for them.
“They are still working on Alex Sandro. He wants to come to Man United. They have negotiate with Juventus over the fee.”
Manchester United finished in second place in the Premier League table last season and ended up 19 points adrift of champions Manchester City.
The Red Devils lost 1-0 to Chelsea FC in the FA Cup final at Wembley after Eden Hazard’s winner.
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