The Portuguese midfielder is understood to have attracted a lot of interest following a series of impressive performances last season.
Manchester United have previously been linked with a move, while last week there was speculation that Arsenal had lodged a bid in the region of £24million.
De Carvalho has denied receiving any proposals for the 22-year-old, but has confirmed that offers have been rejected for a couple of the club's other players.
"There were proposals for Rojo and Slimani, but the values are below what Sporting legitimately want for the players," he said. "For William, there are no proposals."
Southampton are rumoured to be interested in Argentina left-back Marcos Rojo and there have been reports of a £12million bid.
Striker Islam Slimani, meanwhile, is thought to be on the radar of several clubs around Europe after catching the eye during Algeria’s World Cup campaign.