The 2013 Ballon D'Or winner has not played since April 2 after picking up a knee injury and then a subsequent hamstring strain, but has resumed training ahead of the match at the Santiago Bernabeu on Wednesday.
Ancelotti is positive on Ronaldo's chances of featuring and will give him as long as possible to prove his health, but has stressed that he will save the Portuguese superstar for the 'decisive' reverse fixture in Germany next week if he feels that playing him in the home leg would be a risk.
"Cristiano trained normally today, he's trained well in the last few days and tomorrow he's going to have more tests and then we'll make our decision," Ancelotti told a press conference.
"If everyone is everyone is certain he can play without any problems at all, then of course he will play. I am confident he will play, but we're going to make the decision tomorrow."
He added: "If there's any risk of a relapse he won't play. We have to remember that this tie is not going to be decided tomorrow. We may be able to gain an advantage from the home leg, but the really decisive game is next Tuesday in Munich."
Ronaldo is the competition's top goalscorer this season, having netted 14 times in nine matches.