Who gets the nod from Zidane?
It's very unlikely that Zidane will drop Casemiro and Benzema to try some new recipe when his head is on the line. But then he's Zidane.
He's the only man capable of doing something irrational and as unpredictable as he does and still pulling it off by some sort of miracle. So the remaining four players are fighting for one spot.
Gareth Bale: In a Flat 4-4-2 or 4-3-3

Pros: Legitimate second goalscoring threat. He can single-handedly finish attacks. Best suited for quick attacks. Puts in a great shift defensively. Has a point to prove to Zidane, Perez and to himself. Has a great understanding with Modric.
Cons: Could be demotivated to give his all if he's already thrown the towel in.
Lucas Vazquez: In a Flat 4-4-2 or 4-3-3
Pros: Tireless Work rate. Great dribbling and crossing.
Cons: One notch below Bale. Lacks the sync with other attackers. Seldom scores.
Marco Asensio: In a Flat 4-4-2 or 4-3-3

Pros: Wunderkind. Super sub. Serious goal scoring threat
Cons: Relatively inexperienced. Sporadic. Has been inconsistent lately. Doubtful to see him put in 70 minutes or more of elite performance.
Isco: In a 4-4-2 Diamond
Pros: Facilitates possession-based play. Isco's disco. Great at breaking mean and stubborn defensive sides. Provides fluidity in the final third of the pitch.
Cons: Often breaks up the counter-attack by holding onto the ball for too long. Meaningless shooting, often selfish in front of goal only to miscue the chances. Disturbs the structure of the team offensively by dropping in too deep or disjointing a press initiative while defending. Perils of a free-roaming midfielder.
Conclusion

Los Blancos should start in 4-3-3 with BBC up top and Casemiro as the anchor. Given the opponents and their style of play. It makes sense to have players who have the pedigree and know how to keep their heads about themselves and take the game to Klopp's men.
Liverpool is going to come out of the door with their gegenpress that Klopp is famous for employing from minute 1. It makes sense to have Asensio and Vazquez run at tired legs of Liverpool in the later stages of the game.