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.
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