Mata, the club's player of the year in each of the past two seasons, has been unable to hold down a place in the starting XI since Mourinho's return to Stamford Bridge.
But the Special One insists there is no personal issue with the player, and says his continued exile is the result simply of the level of competition in the squad.
|15/4||Basel are 15/4 with BetVictor to beat Chelsea|
"'I have to leave players out because I can't start with 15 players. It's a good problem because I have so many good options."
Mourinho also confirmed Fernando Torres is ready to return when Chelsea take on Basel on Tuesday, having missed the Blues' last three matches through injury.
"Torres is ready to play. He trained with the team yesterday and this morning with no limitation and he is ready to play," he added.
Chelsea can secure a place in the Champions League knockout stages with victory on Tuesday, but Mourinho is adamant it is Basel who should be feeling the pressure.
"Basel are a good team. They won in London because they played better than us (but) we are in a better situation now," he continued.
"If somebody tomorrow has to feel the pressure it's not us, it's Basel.
"Tomorrow we can finish the job to qualify. After the first match, we did our job."