The Spanish international, who joined the Premier League champions from Chelsea at the weekend, has been speaking at his official unveiling his afternoon.
"I cannot wait to play alongside him (Rooney). For me, he is one of the best players in the history of this country," Mata told reporters.
"He is unbelievable, he is a striker that can score, he can assist and he can come into midfield to take the ball and to try to move. I think I will enjoy it a lot alongside him."
Mata has been handed the number eight shirt for United this season.