Mendy received his 4th yellow card of the season in the league match against Atlético Madrid. The Frenchman is undisputable in the starting XI and has the 'Clásico in his sights.
With just one yellow card away from getting suspended, he has a dilemma. Mendy could force the yellow card against Celta to miss the match against Levante and thus play, if Zidane sees fit, against Barcelona.
Everything will change if Mendy doesn't see yellow in the match against Celta. It will then be Zidane's role to decided whether to play him against Levante, with the resulting risk of him getting his 5th yellow, or play Marcelo.
Real Madrid already has a precedent. It happened in the first half of the season. Casemiro could have seen his 5th yellow against Valencia and not play at the Camp Nou, but Zidane left him out and he arrived fresh and with 4 cards.
It is clear, at this stage of the season, that the player chosen to play the 'Clásico' against Barcelona will be Mendy, even more so after Marcelo's errors that led to some of Real Sociedad's goals in the Copa del Rey.