The Portuguese insists he wants his disenchanted No.10 to remain at Stamford Bridge but says he is more than happy to talk to any player who is unhappy at the club
The Blues secured an impressive 3-0 win at Southampton to strengthen their title credentials but the post-match press conference was dominated by questions surrounding the future of the 25-year-old, who walked straight past Mourinho and slammed his chair in the dugout after being replaced after just 53 minutes at St Mary's.
Oscar and Willian came on to set up the ultimately convincing victory but Mata's frustration was clear as he left the field.
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"But my door is open, the club's door is open too so when a player wants to sit with us, we are there waiting - waiting for them.
"But if you ask me: 'Do I want the club to sell him?', I don't want to so it never starts from us, the desire to move a player that we like."
Mourinho did, however, brush off the incident when Mata was substituted by insisting it was a collective frustration rather than an individual one.
"I think his frustration was because of the result," he said. "Because he knew we had to win.
"If we don't win this game the difference to the leaders is bigger so I think his frustration is about this because after the game everyone, including him, was happy in the dressing room and everyone was celebrating.
"So, for me, I have to believe that the frustration was because of the result."
Mourinho also revealed he cannot offer Michael Essien assurances over his involvement in the first team and believes players must work to get into the starting XI on merit by "buying their minutes.
He revealed: "I offer nobody [guaranteed game time]. They have to buy their minutes, I can't promise. I can't offer nothing."