But Mourinho said: “In my project at Chelsea he was a good player. At Man Utd he is a very good player.”
And he said he saw Mata as a key part of his plans.
The 28-year-old had been repeatedly left out of the Chelsea side before his January 2015 exit, completing two full 90 minutes in the Premier League during the 2014-15 campaign.
Mourinho did not feel the Spain midfielder did enough defensive work in a side built around physicality that would go on to become champions – but he has repeatedly claimed that it was Mata who asked to leave.
And at United, Mourinho says, he has a completely different squad.
“At Manchester United I have no players to play transitional football,” said the Portuguese.
“We are not so physically compact like when we won the title in 2015. We are trying to play in a different way that is very adapted to his qualities.
“He has found a natural habitat for his football.”