United have been heavily linked with Carvalho, with Old Trafford officials running the rule over the Portugal international on several occasions this season.
In fact, reports have suggested United scouts have been in attendance at Sporting games 13 times already this term, with Carvalho thought to be the focus of their attention.
But United are not the only club watching the 21-year-old defensive midfielder.
A Bola reports a "transfer shortlist" was left at a Madrid restaurant earlier this week, and a number of names of potential additions were mentioned.
Carvalho was one of the names, along with Liverpool's Luis Suarez, Sevilla midfielder Ivan Rakitic and defender Alberto Moreno, Zenit star Axel Witsel and Marseille playmaker Mathieu Valbuena.
United remain the most serious suitor for Carvalho, as evidenced by the club's continued presence at Sporting Lisbon games but it looks like they will have strong competition for his signature.