Spain international goalkeeper De Gea has been in superb form for Louis van Gaal's side this season and his performances have reportedly put him on the radar of European champions Real Madrid.
However, Mendes stated that last week that he expects De Gea to remain at Old Trafford and it appears the former Atletico Madrid goalkeeper could be about to commit his future to United.
"It's not true that the deal is done," Mendes told Spanish radio station Cadena Ser. "I am negotiating his contract renewal but we haven't signed anything.
"As of the moment, we'll see what happens.
"The players are the ones who decide what they want. We're holding talks and as of the moment it seems that he could continue with United but as I say, we haven't signed anything yet."
De Gea is in his fourth season at United and has made 118 Premier League appearances for the club in that time.
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