Sanches has been heavily linked with a move to Old Trafford in the summer.
And according to the Manchester Evening News, United scouts watched the youngster as Benfica were knocked out of the Champions League following a 2-2 draw with Bayern Munich.
The 18-year-old has emerged as one of Europe’s hottest properties this season.
The midfielder only made his debut for Benfica last October having spent the previous season playing for the club's B team in the second tier of Portuguese football.
Renato Sanches - 5 things you need to know
Sanches has a release clause of around £62million but Benfica are said to be willing to sell for closer to £47m.
However the Red Devils face competition for the midfielder, with Arsenal also interested in his signature.
The Gunners were also apparently at the Estadio da Luz last night to watch the teenager.