The Portugal international has a release clause in his contract at Sporting Lisbon and will leave if anyone triggers it.
Sky sources understand he is one of the targets United’s new boss Louis van Gaal is considering as he looks to add another midfielder after signing Ander Herrera from Athletic Bilbao last week.
Sporting are prepared for a bid but have warned they will not allow the Portugal international to go on the cheap.
Director Augusto Ignacio told Sporting TV: “It’s not like the old days when players left Sporting at any price.
“We are not concerned with William Carvalho because we have alternatives who will want to play as well.
“The termination clause ensures a great deal for Sporting. I am very pleased with the negotiations.”
Carvalho, who played for his country at the World Cup, has been linked with a host of top European clubs this summer including Real Madrid and Juventus.