After Costa was rested against Schalke 04 on Wednesday, Mourinho insisted that the forward couldn't play three matches in a week, but the Portuguese coach has admitted that the in-form Spaniard will play at the Etihad Stadium this weekend.
The 51-year-old told reporters: "Everybody is ready to play. Diego [Costa] is not in the best condition but he will start.
"If he plays, he won't play against Bolton on Wednesday."
Costa has scored seven goals in his first four games in the Premier League.