Mourinho confirmed on Friday that Mata is closing in on a £37m move to Manchester United, but while the Portuguese coach admitted that he would be sad to see Mata leave, he is happy that the Spaniard has earned a "fantastic" move.
The 50-year-old told reporters: "It is a good offer. Respectful to us, respectful to the value of the player. That allows us to try to bring in another player because we need another player.
"I asked him when the offer comes, are you really happy to go? He was saying that he was. After that, we could feel mixed emotions.
"We are happy that he goes to a fantastic situation, but we are all sad as well and some of him was a bit of the same."
Mata had featured 17 times for Chelsea during the current campaign, but many of his appearances have come from the substitutes' bench.