Real Madrid has often come from behind but they cannot afford to take it lightly this time as this is the first time the new format has been introduced, so if they don't take it seriously then they might just get knocked out. I agree that Ancelotti should have started thinking about benching Mbappe because besides being an option, it might make Mbappe better. There are times when a player can be better when he is left on the bench because he will reflect and try to improve himself to become a starter again.
When you say that, it's as if Mbappe has been the one making Real Madrid's game unusual and I don't think it's wise to make such an assessment. I don't think there's anything wrong with Mbappe, it's just that he still needs time to adapt.
Also remember the team's game as a whole, their problem is not just in the front line, but it's the whole thing. They have problems in defense and they still have problems in midfield, that's what makes Real Madrid's game messy so it's very difficult for them to get a win. So I disagree with what you said.