Sporting CP coach Silas insists Bruno Fernandes is being undervalued.
Reports in Portugal on Thursday claimed that the two clubs remained some way apart on the valuation of the player. United won't go above paying £42million up front for Fernandes, while Sporting demanding at least £55m for their captain.
"He is an extraordinary player, the best and most valuable in the league, as he was last season," Silas said.
"I've heard someone say that he's not worth the money you are talking about and it's true, he's worth a lot more.
"In addition to attacking he also sacrifices himself to defend. Bruno is the best and the most valuable. When we have such a player it's normal for everyone to want him."