It doesn't make sense that Manchester City is so fragile just because of Rodri's absence, but the absence of Rodri has caused Manchester City to lose a lot and they can't find an alternative to replace him. Real Madrid is the absolute favorite for the return match. Real Madrid's European success has always been admirable and they can continue this every time, there is no other team that can do this.
Although Manchester City's modern, fast and attack-oriented game will still make Real Madrid difficult, Real Madrid will use this as an advantage due to their defensive weaknesses.
Manchester City is fragile because of Rodri's absence in real sense. Guardiola announced much earlier that Nicon Gonzalez will help them plug in the gap which is left by the Rodri's absence but truth of the matter is that Rodri's absence will continue to be left out until a more stronger midfield takes in the gap. Pep Guardiola didn't do so much in terms of the back up players. Ancelotti have done well in working hard on his backups unlike the Guardiola team who is battling for survival.
That's only one little fraction of any logic explanation that could give reason to the downfall of Manchester City this season. If it was one player that pulls down the whole team due to his absence, in my opinion that would be another weakness of any top team. There can be dependencies and there can be a weakening due to one player missing for a long time, but Manchester City turned into a wreck of a team compared to what they were once able to play and to dominate almost any other team, no matter which team it was from the top list of European clubs.