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.
