Metro reports that United are pondering a move for the 22-year-old as a long-term successor to Patrice Evra.
United boss David Moyes’ attempt to prise away Leighton Baines from Everton is appearing to stall.
Evra’s deal at Old Trafford is set to expire at the end of the season and the defender has been strongly linked with Paris Saint-Germain and Monaco.
Moyes watched Sandro in action in Porto’s 2-0 defeat at Atletico Madrid last week.
