Rooney, 28, is now in talks with United to extend his current deal.
Spain forward Mata, 25, said: "For me, he's one of the best players in the history of this country. He's an unbelievable player. "Mata, who has signed a contract understood to run until summer 2018, could line up alongside Rooney for Tuesday's Premier League game at home to Cardiff City."He's a striker who can score, assist and come into midfield to take the ball," added Mata of the England international. "I will try to connect with him as much as possible and find the gaps in rival defences. I think that's my best style of play, to get into positions where I can assist the strikers."Mata, who will wear the number eight shirt, also said United's never-say-die attitude meant they could still retain their league title. "What I like most about this club is the character," said Mata, who was twice Chelsea's player of the year. "I think if another club were in this position in the league, it would be very difficult to come back and take first position, but this club can do it. "This is the real image I have from Manchester United, always fighting to win titles, always coming back from difficult moments. "The former Valencia player scored 32 goals for the Blues and made more than 130 appearances but had failed to command a regular starting place under Jose Mourinho, who returned to Chelsea last summer for a second spell as manager. Mata, capped 32 times by World and European champions Spain, admitted moving to Old Trafford was "really important" to his hopes of making his country's World Cup squad. Manchester United boss David Moyes said he was surprised the club was able to sign a player of Mata's calibre during the January transfer window. "I am thrilled," said Moyes, whose team are seventh in the Premier League, 14 points adrift of leaders Arsenal. "I got wind a month or two ago it could be possible, but I never thought we could pull it off."When we got the sniff there was a chance Juan could be available we went after it right away. There will be more players like Juan in the future. I've no doubts about that."