Mourinho said: “Things are going in a good direction. We have allowed Juan to travel and have the medical.
“The player deserves respect and to play and because the offer is right, we allowed him to negotiate.”
The 25-year-old Spain international is set to arrive in Manchester on Friday to have his medical.
Mourinho also revealed that Basel midfielder Mohamed Salah will join Chelsea from Swiss champions Basel in an £11m deal, subject to a medical and agreeing personal terms.
The 21-year-old would become their third signing of the January transfer window after the arrivals of Nemanja Matic and Bertrand Traore.
“We know he wants to come, he told me personally but now it is terms and medical he has to pass,” added the Blues boss.
Mata, who cost Chelsea £23.5m from Valencia in 2011, failed to command a regular starting place at Chelsea under Mourinho, who last summer returned to Stamford Bridge to start a second spell in charge.
The Spanish midfielder, who has won 32 caps, said goodbye to team-mates and staff at Chelsea’s Cobham training ground on Wednesday.
Mourinho has trusted Eden Hazard, Oscar and summer signing Willian in attacking roles this season and while his relationship with Mata is healthy, the Portuguese conceded Chelsea’s system does not suit the player.
United denied having made a bid for the playmaker, but it is understood they have made an offer through intermediaries working on the deal.
Mourinho added: “I asked him when the offer came whether he really was happy to go and he said ‘yes, I am really happy to go’. After that we could feel mixed emotions.
“This is a fantastic opportunity for him. Another big club wants him in the country he want to stay in, has made the right offer for him, so, although lots of clubs feel you should not sell to rivals, we are different.”
On Tuesday, Chelsea insisted that Mata, who had been voted the club’s player of the year two seasons in a row, was not for sale.
But the player, who was part of the Spain squad which won the World Cup in 2010 and European Championship in 2012, has been substituted in nine of his 13 Premier League appearances for the Blues this season.
Mourinho added: “I think this club is a special club and it cares for people. It is very respectful of its players and especially those who wrote history and Juan is one of these.
“He was fantastic for Chelsea and Chelsea was fantastic for him. We are not afraid he is going there to do very well. We want him to go there and be happy.”
Meanwhile, the Blues boss criticised his Arsenal counterpart Arsene Wenger for saying that Mata’s proposed move to United raises questions about the fairness of the January transfer window.
Mourinho said: “Wenger complaining is normal, he always does. But when a player plays for a club on 20 August and then on 5 September plays for a different club this is normal. This is the market.
“If Wenger sold Mesut Ozil to Manchester United I would be happy because they are losing a very important player, like we are with Juan, but this is his nature.”
Written by Steve Milne
