Mourinho confirmed United have broken their transfer record - a ?30.75million deal for Dimitar Berbatov in 2008 - to recruit the 25-year-old, who was Chelsea's player of the year in each of the last two seasons.
The Chelsea boss said: "Things are going in a good direction and we allowed Juan to travel and to have the medical with United. I think at Manchester.
"Things are really going in that direction. 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.
"It's good for Juan, it's good for the Premier League and the offer in financial terms, is the right offer.
"The offer, compared with what the club paid two years ago (when Mata signed from Valencia), was fantastic.
"If Man United pays their record for a player it's because they know exactly what they want.
"They are convinced he's going to be important for them and we hope that he does very well for them. No fears at all. We are convinced he's going to do well.
"When we play him we are going to try to win and he does the same."
Source : PA