The 24-year-old has been in outstanding form for his club this season, attracting attention from clubs across Europe.
However, A Bola reports that United have held informal talks with the Lisbon outfit about a potential move and are in pole position to land him.
The main competition is likely to come from Italy, where Fernandes played from 2012-17 for Novara, Udinese and Sampdoria before returning to his home country in 2017.
Milan are understood to be showing particular interest in the Portugal international, who has a deal until 2023 with Sporting.
Fernandes has a €100 million release clause in his contract (around £86 million) and has scored 27 goals in 45 games in all competitions this season.
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.