He might have been, but shit things happen in football. He's injured and isn't an option right now. Clubs have to plan for possible scenarios and have options to try different formations or systems, as situations adapt, too.
We will seriously come undone if we rely on those three for that position for the rest of the season. This is also taking into account everyone being fit and in form. If Casemiro's form drops or if he has a lengthy injury, you're relying on McTominay and Mainoo for that spot with no further options. We're asking for trouble by not exploring more players for that position. I'd rather have a player in the squad who could fulfill that role than have a desperate looking bench.