Mata admits maintaining the standards set by former boss Sir Alex Ferguson is almost impossible, but has still backed United to return to the top of the game.
He said: "All he has done here, everything Ferguson has accomplished is pretty much unattainable.
"He has won everything possible to win with this club in the last 20-25 years and it's tough to maintain such a level of excellence.
"But at the end of the day, it was these players that won the league last season. And the new manager has arrived knowing what this club is.
"I think he (David Moyes) has a great relationship with Sir Alex Ferguson and with time everything will get better, the results will get better and we'll get back to being where this club deserves to be.