Reports claim that United are monitoring the Brazilian international as they continue their search for a replacement for Patrice Evra.
Evra is expected to leave United when his contract with the club expires at the end of the season.
Portuguese daily Record reports that United are ready to join Inter Milan in the race to sign the 22-year-old.
Inter Milan have reportedly already had a £12.5million bid for the defender turned down as Porto is holding out for more.
Sandro joined Porto from Brazilian side Santos for £8million in 2011.
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The start of the 2017-18 Premier League season is just around the corner and is shaping up to be one of the most competitive yet.
As always it is the so called “big clubs” that have splashed the most cash in the transfer window so far this summer. Manchester United or City? Arsenal or Tottenham? Who are the winners and losers of the transfer window so far?