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United are expected to complete a club-record move for the Chelsea midfielder on Friday once a medical has been completed in Manchester.
And speaking at his Friday press conference Mourinho wished Mata well and confirmed the deal should go through without a hitch.
He said: "Things are going in a good direction. We allowed Juan to travel and have a medical and I think sooner rather than later things will be finalised.
"He's a fantastic player and despite not playing a lot for us in the last period could be a very important player for us. But Juan had this fantastic opportunity for him.
"It's what he wants. It's what he asked us to accept. Chelsea Football Club is proud to act this way, in an open way, in a human way."
United will reportedly hand over £37million for the Spain international, far exceeding their previous club record of £30.75m they spent on Dimitar Berbatov in 2008.
The fee also represents a hefty profit for Chelsea on a player they signed for £23.5m from Valencia in 2011.
Mata was named Chelsea's player of the year in each of the past two seasons but has been reduced to a bit-part role since Mourinho returned to Stamford Bridge.
"The offer we've received we feel is the right offer for the quality of the player," Mourinho said. "It's a good move for Juan, it's a good move for the Premier League and the offer is the right offer. He was fantastic for Chelsea, and Chelsea were fantastic for him."
Responding to criticism of the sale from some supporters, he said: "If the fans are sad that Juan is leaving, I will say that we are also sad. No one is jumping around the place."
Chelsea announced on Thursday they are close to securing a replacement in the shape of Basel's attacking midfielder Mohamed Salah.
Salah has still to agree personal terms, but Mourinho was effusive in his praise for a player who twice scored against his side in this season's UEFA Champions League.
He said: "I like how he can play the same way our attacking players do, adapting to the left or right, or as a No 10.
"He's fast, he's creative. We believe if he comes, he will need some time to adapt - like Willian did."
