Isco stood out with two goals including a last-gasp victor in Saturday's game, but he has been used sparingly in Europe this season, getting just 77 minutes in total across Madrid's nine Champions League games to date.
Defending champion Madrid defeated Bayern 2-1 last week in Munich.
When asked about who would play alongside Cristiano Ronaldo and Benzema on Tuesday, Zidane said he had; "already decided", who would start, "but I can't say any more than that", he added. "He wants to come back, he has been working hard".
The French manager expects Bale to be recovered for El Clásico but he has confirmed that the Welshman won't play against Bayern Munich: "Gareth has worked a lot to go back with the team after his injury. We will see how he progresses day by day". Los Blancos, seeking their first league title since 2012, have a three-point lead and a game in hand.
FILE - In this file photo dated Saturday, Feb. 18, 2017, Real Madrid's Gareth Bale celebrates after scoring his side's second goal against Espanyol during a Spanish La Liga soccer match between Real Madrid and Espanyol at the Santiago Bernabeu stadium in Madrid.
Bale missed Real's 3-2 win at Sporting Gijon Saturday and Zidane was far from confident he will be fit in time to face Barca.
The Wales forward was out for almost three months earlier this season after a right ankle injury.
AC Milan rescue draw against Inter in injury time
Grabbing attention in the mid-table with seven matches to go this season, Inter and Milan will be playing for nothing but pride. Elsewhere, Cagliari beat Chievo Verona 4-0, Torino drew 1-1 against Crotone and Bologna's match at Palermo ended 0-0.
Zidane refused to reveal who he will replace the Real Madrid number 11 with, however, a recent spat of impressive performances, make the inclusion of Isco a strong likelihood. Marco Asensio and Alvaro Morata are Zidane's other attacking options.
Real Madrid's struggling defence will have to keep Bayern Munich's deadly striker Robert Lewandowski at bay in the Bernabeu today to reach the Champions League semi-finals.
Zidane admitted it was "surprising", Isco had played so little, but said there were more important games before the end of the season. It's not over yet; they've got quality players that are risky.
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.
By contrast, Bayern welcome back their main man in Lewandowski, who has scored 38 times in 40 appearances this season, from a shoulder injury.
"Lewandowski is an excellent player but it isn't all about him", Casemiro said in a media seminar.
"Obviously he is a very important player for us", said Bayern captain Philipp Lahm.