De Gea has been in sparkling form for Louis van Gaal's side this season, keeping eight clean sheets in all competitions.
With a year remaining on his contract, De Gea's performances have led to talk of a big-money return to the Spanish capital,
However, the former Atletico Madrid man's representative is confident De Gea will reaffirm his commitment to United.
"He [De Gea] has one year left on his contract and I think that he will stay in Manchester," Mendes said in an interview with BBC Sport.
"The player will decide, but he's happy there, at the moment."
But Mendes is unsure of the United future of another of his clients, striker Radamel Falcao.
Falcao has made just nine Premier League starts since joining on loan from Monaco, although Mendes expects the Colombian to sign with a top European club if he does not make his switch to Old Trafford permanent.
Mendes added: "If he [Falcao] will not stay in Manchester, he will play for sure in one of the biggest teams in Europe."
