The 25-year-old's move to Old Trafford is expected to be completed on Friday, after a delay due to Chelsea's negotiations with Mohamed Salah and poor weather conditions.
Speaking at his pre-match press conference ahead of the game against Stoke in the FA Cup, Mourinho confirmed Mata was heading for United and said he is happy to join David Moyes' men.
"We allowed Juan to travel and have a medical at Manchester United," he said. "Sooner rather than later things will be finalized. I asked him when the offer came: 'Are you really happy to go?' He said 'I'm really happy to go'
"He is a fantastic player. In spite of not playing much for us, I still think he could be important for us. It's a good move for Juan, it's good for the Premier League and the offer is the right offer. With the two deals, Juan and Kevin de Bruyne, we put ourselves in a very good position in relation to financial fair play."
Mourinho is not concerned about letting Mata join a domestic rival, and hopes the Spaniard is a success at Old Trafford.
"Many clubs, managers, boards feel you don't sell to a rival club, we feel and we think different,” he added. “We are not afraid of him doing well at Man United, we want him to be happy and do well at Man United. He was fantastic for Chelsea, and Chelsea were fantastic for him. When he comes back to Stamford Bridge with a Man United shirt on, for sure we will show how much we respect him
"We feel mixed emotions... we are all sad and I think with him it was a bit the same."