The boss praised the Spaniard's impact in Saturday's 1-1 draw at Tottenaham, saying he "changed the team", before accusing Jan Vertonghen of acting for Fernando Torres' red card
Mata has often been overlooked by Mourinho during the start of the campaign but came off the bench at half-time on Saturday to inspire a turnaround in the game and set up John Terry’s equaliser.
"I think this is the way players have to say 'I want to play'," said Mourinho. "'Blah, blah, blah' is not good, conversations with you [the media] is not good, agents talking is not good.
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"I’m a very happy man to say 72 hours in advance he will play against Steaua Bucharest. He earned that on his own."
Chelsea looked the more likely winners at White Hart Lane before striker Fernando Torres was sent off for a second booking following a clash with Spurs defender Jan Vertonghen.
Mourinho was incensed with Vertonghen’s reaction to the challenge and accused the Belgian of play-acting to get Torres dismissed.
The Blues boss also suggested that the centre-back should have been suspended for the game anyway after pulling down the shorts of Aston Villa forward Nicklas Helenius in the Capital One Cup on Tuesday night.
Mourinho added: "I learned even before I arrived here what the game is in England and the principles you have to play. Things like this in other countries people say ‘the guy is very intelligent’ but not in England.
"That’s why I don’t think the referee is guilty. He made the wrong decision but the referee trust the player. The tendency is to follow when the player holds his hands to his face like a violent action.
"The referee had very good intentions but in that situation it looks like the player needs to go to hospital with a broken bone until you see the TV replays. To push the referee for a bad decision is something I really don’t like. Maybe I'm old fashioned.
"[Vertonghen] is a fantastic player but he’s a special guy. Three days ago he left the Aston Villa player naked. He should have been suspended. The boy was naked, it’s ridiculous - you go on YouTube and it’s top of ridiculous videos."