Speculation increased following Mata's angry reaction to being substituted against Southampton, and Mourinho says such tactical decisions "hurt" him and only wants the best for the player.
"If people think they can get our good players on sale, sales [have] finished in London," Mourinho told reporters. "I keep saying that I want him to stay. There [are no] offers. We have no offers."
"I try to do my job forgetting I'm a soft heart, pretending that I'm a hard heart. Many times it hurts me, not just with him, with decisions I have to make.
"But I think to do my job in the best way ... thinking the team [is] more important than any player. When I have to make a decision I always analyse the situation and try to make the best decision for the team ... putting the person secondary.
"It hurts me. He's a good kid, he works hard, not an easy situation for him not to be playing every time. He started nine matches in the Premier League, had lots of minutes on the pitch. I love it if he does well. That's what I want."