The Poland worldwide missed the initial leg of the Champions League quarter final tie because of shoulder injury but is likely to go back to the starting XI in Spain.
He was also asked about Sergio Ramos, Tony Kroos and Luka Modric, who would miss a semi-final first leg if they are booked against Bayern, but said Madrid would not be "thinking about who might get booked", when the most important thing was to qualify for the last four.
"He will not play because we don't want to take any risks", Zidane said on Monday.
Madrid had most of its regular starters available for the loss to Celta in the Copa del Rey quarterfinals, where it couldn't come back after conceding two goals on counterattacks in the second half.
Ronaldo will be well rested after he was left out of the squad for Los Blancos' 3-2 La Liga win over Sporting Gijon last weekend (a precautionary measure as he was one domestic booking away from missing the El Clasico against Barcelona on Sunday). "We will see how he progresses day by day".
"It'll be a great game", said Bayern coach Carlo Ancelotti, who led Madrid to the 2014 title.
They beat Sporting Gijon 3-2 on Saturday but conceded two sloppy goals, needing a 90th-minute victor from Isco to help them keep Spanish title rivals Barcelona at arm's length.
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Bale has struggled with injuries this season, having missed three months earlier in the campaign because of ligament damage in his right ankle.
Zidane did not say who will replace Bale, but Francisco "Isco" Alarcon - who scored twice to lead Madrid to a late 3-2 win over Sporting - is expected to start.
If Bayern Munich is to have any chance of a sixth straight semifinal appearance in the Champions League it will have to do something that only one team has achieved this season - defeat Real Madrid at Santiago Bernabeu Stadium.
Bayern's preferred centre-back duo Jerome Boateng and Mats Hummels face a race against time to be fit, while Javi Martinez is suspended after being sent-off in the first leg.
"It must be tough for him to have only played 77 minutes in the Champions league but we've got a lot of important games and we'll see what's going to happen tomorrow". We'll try to play our game and prepare well the game, like the first leg. This result is not definitive, so we need to play another great game.
Cristiano Ronaldo became the first player to score 100 European goals last week - and the Real Madrid forward is closing in on even more milestones in his impressive career.
"Lewandowski is an excellent player but it isn't all about him", Casemiro said in a media seminar. We know Lewandowski is a real No. 9, the player he is.