Brazil playmaker Oscar scored a sublime free-kick in Sunday's 1-0 fourth-round defeat of Stoke as Mourinho celebrated his 51st birthday with a win a day after Mata's transfer to Manchester United was completed.
Mourinho, who read Mata's departing open letter to Chelsea fans, said: "I feel sorry that I didn't make him happy, but football is football and I build a team around Oscar in that position and on the sides the other people is doing very, very well.
"Juan is not comfortable on the sides. He tried very hard and he did very well in some matches playing on the sides, but it's not his natural habitat.
"So when a club like Man United comes and for sure they told him he's going to play in his favourite position and he wants to go, he goes."
Chelsea moved quickly to replace Mata and completed the signing of Mohamed Salah from Basle on Sunday on a five-and-a-half year contract.
Salah joins a squad performing well and with Oscar as the attacking focal point.
Mourinho praised Oscar's work ethic and the performance of his team.
The Portuguese said: "It's always good when you see a player that, after the normal training session, dedicates some minutes every day to train a specific thing. To score a beautiful goal was good.
"The 1-0 doesn't reflect the quality of our game and the dominance of our game.
"We hit the post twice. (Stoke goalkeeper Asmir) Begovic again showed how good he is. The boys played really well.
"We controlled the game since minute one and I'm happy not just with the victory, but also the performance."
Stoke manager Mark Hughes was magnanimous in defeat.
"Overall Chelsea on the day deserve to go through, but I was pleased we were very much in the game right till the end," Hughes said.
"You have to get a little bit of luck when you come to places like Stamford Bridge and maybe we didn't force the issue enough to change things in our direction.
"You're never happy to go out of a cup competition, but in terms of performance I'm reasonably pleased.
"We've got a huge game on Wednesday now so we'll pick ourselves up and go for that one."
Stoke could have Peter Odemwingie in the squad at Sunderland, with Hughes expecting a swap deal which will see Kenwyne Jones go to Cardiff to be completed on Monday.
He added: "It should go through tomorrow and will be concluded then.
"We're pleased to get Peter in. He's a good Premiership player with experience at this level, a goal in him and a bit of pace - which we need to add to the squad."
Source : PA