Diego Costa sustained a minor muscular injury on Wednesday and has missed two days of training, but the Chelsea boss has not ruled out including him on the bench against Everton.
Mourinho told reporters on Friday that he remains open to the possibility of Costa featuring at Goodison Park despite the fact that his €37.5 million summer signing has had to miss two days of training.
"He is a big doubt, let's see," Mourinho said. "Everyone else is fit.
"We are trying everything for him to play [on Saturday]. He didn't train for the last two days. Today is the last day.
"We have to try to see if the reaction is positive. If the reaction is positive, we'll take him and select him on the bench."
Costa has made an impressive start to his Chelsea career, scoring two important goals in his first two Premier League matches against Burnley and Leicester City.