Sporting coach Marcel Keizer admitted on Wednesday that Fernandes could leave the club this summer.
Fernandes also commented publicly that any move was out of his hands.
Reports in the Portuguese press have suggested United aren't close to Sporting's valuation of the 24-year-old.
United are 'irritated' about the reports and believe Sporting are trying to whip up an auction, according to Manchester Evening News.
The Red Devils have not lodged a bid for the Portugal international, although they are admirers of his talents.