The Portuguese forward has missed Real’s last three games – including the goalless draw in the first leg at the Etihad Stadium last week – with a thigh injury.
But he has played a full part in training ahead of Wednesday’s Bernabeu clash and Zidane says he is ready to return.
Speaking at his pre-match press conference, Zidane said: “Cristiano is okay. He is 100 per cent. He trained this morning and he will be there tomorrow.”
Zidane insisted there was no risk attached to playing Ronaldo, who has struck 16 times in 10 appearances in the Champions League this season, and is the competition’s record goalscorer.
The Frenchman said: “The whole week, little by little, Ronaldo has been recovering. That’s how it is with this player. He’s very quick, he looks after himself a lot, and tomorrow he will be 100 per cent.”
But he delivered less optimistic news about two other key players, as the Real boss confirmed both forward Karim Benzema and defensive midfielder Casemiro would miss out.
Benzema was withdrawn at half-time in Manchester with a hamstring injury and Casemiro has since suffered a blow to the hip.
Zidane said: “Karim and Case are different. Karim still had a doubt and we don’t want to risk it, and the same goes for Case. It is not an injury, just a bit of a problem.
“But we have another squad list and so everybody else can play.”
The World Cup winner smiled and said: “I think they have had a wonderful season. [Claudio] Ranieri, their coach, did a wonderful job. I am really happy for them.
“It is the first time they are champions – it is something to be admired very much. For me it is no miracle – it is what this team deserves.”