The Belgium international is being linked with a move away from the Lisbon giants, but his current club insist a transfer is still far away despite interest from the Red Devils
The Belgium international has reportedly attracted the interest of AC Milan and Real Madrid, but Benfica are keen to hold on to the versatile midfielder.
Sir Alex Ferguson is also thought to be an admirer of the former Standard Liege player as he looks to bolster his midfield ranks at Manchester United.
"Witsel is a player of Benfica and he is doing the pre-season with us," Rui Costa stated to reporters.
"We are only thinking about the Eusebio Cup [against Real Madrid] and not about transfers. Every player who is in pre-season training belongs to Benfica and there are no ongoing negotiations."
Meanwhile, Emilio Butragueno, who was also present at the press conference, was coy about Los Blancos' alleged interest in the Belgian midfielder.
"We will see. Benfica are a great side, but I am not here to talk about transfers. I'm here to thank the fact that we will contest the Eusebio Cup."
The 23-year-old has a contract with Benfica that runs until the summer of 2016.