The England international was encouraged by the Red Devils' performance in the draw at Arsenal and expects his new teammate to become a huge player for the club once settled.
Mata has started four games since making the move from Premier League rivals Chelsea but has tasted victory for his new side only once.
In the middle of what has been a rocky season for the champions under the stewardship of David Moyes, Rooney has been impressed by the Spaniard. However, he warned Mata will need time to find his best form alongside himself and fellow striker Robin van Persie.
"It's exciting. Juan has come in and it's another world-class player to have in the squad," he told the club's official website. "It'll take time — it can always take a little bit of time for every player to gel and to bond but we really hope we can all start scoring goals and help the team be successful."
After Wednesday's goalless draw with Arsenal, United sits seventh in the Premier League — 15 points behind leader Chelsea. Rooney has stated that the title is likely a bridge too far for the reigning champions, but the England international was encouraged by United's performance at the Emirates Stadium.
"I thought we did all right," he said. "It was a decent point. Arsenal are obviously a good team but we defended really well and maybe could have nicked it with the couple of chances we had.
"Coming to Arsenal any time and getting a point is always a good result. You have to take confidence from it. We've been stop-start for most of the season, but we hope we can build on this. We have a bit of a break before our next game now and hopefully we can prepare well and kick on for the big matches coming up."