Mata has made just five starts in the Premier League this term, from a possible 12, and has scored only once in all competitions, against Arsenal in the League Cup.
Reports have suggested a January return to his native Spain could be on the cards for the 25-year-old, as he looks to boost his chances of making his country's FIFA World Cup squad, Atletico Madrid mooted as a likely destination.
Mourinho praised Mata's attitude and application, having previously claimed his work-rate was a reason for his omission, but hinted that he is likely to continue to struggle behind Eden Hazard and Oscar in the Chelsea pecking order.
"There is one thing that is obvious, which is I can't change the rules and I can't start the match with 12, 13 or 14," Mourinho said.
"I'd love that because many people deserve to play and Juan is one who deserves to play because of the way he works, behaves, the way we see him every day.
"But if I play him and I don't play Hazard or Oscar, you are asking me about Hazard or Oscar in this moment.
"It's a consequence of the squad we have where, for these three positions behind the striker, we have so many options.
"It's hard for me. I don't have a special pleasure leaving players out. I enjoy playing them and making them happy, but there's nothing I can do."
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