The Sporting chief said the club is not looking to sell any of its first-team stars but conceded that some of them have release clauses that mean it's difficult to guarantee their future.
Manchester United have scouted Carvalho on several occasions this season, and the £37m-rated midfielder has impressed on every occasion.
He found the net for the fourth time in the league this season at the weekend, a neatly taken goal that showed off his considerable power and technique on the ball.
United have been lacking a dominant midfield presence this season, and Carvalho would fit the bill although he would be extremely expensive.
As reported by A Bola, de Carvalho said: "I'm not counting on anyone leaving, but remember that there are termination clauses that we all know [about].
"I have nothing to say about a situation when someone comes along to meet the clause, but we have no way out."
United boss David Moyes is expected to be extremely active in the transfer market this summer, with several reports claiming the Glazers will give him a massive summer budget to strengthen the playing squad.
Central midfield is a position in need of reinforcements, and at 21 years old, Carvalho fits the profile of a United signing.