He is irreplaceable. Literally and figuratively. because right now Madrid have no one to replace Casemiro with. His natural replacement was Llorente, but he did not enter into Zidane's plans. The position was inherited by Fede Valverde, but he is injured.
The game against Levante was the clarifier. Without Casemiro, Madrid are lost, they get stuck and stop being the fearsome rivals that they are when the Brazilian is on the pitch.
What Madrid will be wondering at the moment is what will happen when Casemiro is missing. If Levante put Madrid under pressure at Bernabeu, what will happen if Casemiro is missing in Paris or Sevilla?
Real Madrid are waiting for two, more than demanding matches this week, and with Casemiro dragging the fatigue of the international break, Madrid will pay the price of not having a reliable replacement for their midfielder.