"As a team-mate and a friend I would like him to stay and be happy because he is a very good player for us."
Mata is close friends with United goalkeeper David De Gea, who Sky Sports News understand is on the verge of signing a new long-term contract with the club that will make him the best-paid goalkeeper in the world.
De Gea has been United's player of the year on a record four occasions since arriving from Atletico Madrid eight years ago, and Mata believes it is vital he remains at Old Trafford.
The 31-year-old, said: "David (De Gea) is the best goalkeeper in the world and one of my best friends so I want him to stay.
"On a professional level, it would be great if he stays because he will give us many points over the next season, so I'm very happy if he stays."
Mata believes United must improve on last season where they finished in sixth place in the Premier League, which was 30 points behind champions and bitter rivals Manchester City.
"This club has won more trophies than any other club in England and we want to win again big trophies. That is what the history of this club and the fans deserve.
"We know we have to improve and have a better season than last season. I think we have very good people in the changing room and good people in the manager.
"We want to improve, we want to win and we are working towards that."