Ronaldo has recovered from a hamstring injury which forced him to miss Madrid´s last three games in all competitions and has resumed full training.
But striker Benzema is yet to recover from a hamstring problem and midfielder Casemiro is still missing after a knock to the hip in Saturday's 1-0 win at Real Sociedad.
"Cristiano is fine, he is 100 percent. He has trained and will be with us tomorrow, there is no risk," Zidane told a news conference.
"We need a 100 percent Cristiano," he said. "He's a different player. His numbers show that and I'm happy he's with us tomorrow. I'd have liked Karim and Casemiro to be there too, but others will play and that's that," added Zidane.
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Forward Lucas Vazquez replaced Ronaldo in the goalless first leg in Manchester and is expected to fill in for Benzema.
City midfielder Yaya Toure has recovered from a thigh injury but Pablo Zabaleta and David Silva have been ruled out.
BEST FORM
Real are in their best form of the season, having won 10 league games in a row. But Zidane, who took charge in January after Rafael Benitez was sacked, said not reaching the Champions League final in Milan on May 28 would be a failure.
"Until now we have said we haven't achieved anything. We are in good form, but our aim is to do everything possible and try and go through.
"But it will be very difficult. If we don't get through, it'll be a failure."
While the 10-times European Cup winners are seeking to reach their 14th final in Europe's elite competition, City are aiming for their first in the Champions League. They last reached a European final when they won Cup Winners' Cup in 1970.
“The story of the two clubs is very different, for me, it would only be a failure if we did not play to our potential,” said Manchester City's Manuel Pellegrini, who coached Real in the 2009/10 season before being sacked.
“We will try tomorrow to play in the way we have played every week,” he added. “We tried to do it in Manchester but we did not have a great day, tomorrow we will try to press and attack the ball from the very first moment.”
(Editing by Hugh Lawson and Ken Ferris)
