Mata was the highest-profile transfer during January, moving north from Chelsea for £37.1m.
Although he is yet to score for his new club, the 25-year-old midfielder has made an impressive start to his career in a red shirt.
And Rooney is pleased to have Mata in the United squad, saying: "It's exciting.
"Juan has come in and it's another world-class player to have in the squad.
"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."
United are heading to Dubai for warm-weather training and Rooney added: "It's nice to spend time together.
"When you're at home you go to training then you go home and see your families.
"But with trips like this you're training then spending time with the lads. It's good for us all to be together.
"It's a different environment for us and hopefully we can get some good training sessions in so we come back ready for the Palace game and the start of the Champions League.
"We've got a lot of massive games in the Champions League and the league.
We're all looking forward to them."