Their midfield was key last season and if we now think about who is left of that midfield, both are gone. Gundogan and De Bruyne. Since the two controlled the whole Manchester City game and also scored goals and gave countless assists, it doesn't come as a surprise that now when both of them are absent the game is much worse than it was last season. I think this is probably the difficult task ever for Guardiola to find two successors for Gundogan and De Bruyne who are able to manage the Manchester City game on the same level.
I don't think Manchester City achievement last season winning Champion League tittle depend both them only between Gundogan and De Bruyne but also have more important key midfielder Rodri. I think Manchester City not matter when losing Gundogan or De Bruyne but not for Rodri, in this season Manchester City performance stable on the winning trend when Rodry playing but not guarantee if there are have De Bruyne Manchester City will easily win every match.
Losses Gundogan not much problem for Manchester City in this season and De bruyne absent can back up well by Rodri, I think Manchester City must find the same level players when Rodri have absent.
Regardless of who plays the role of key player in the Man City lineup, the biggest factor that most influences the team performance is Pep Guardiola. I think many people would doubt City's 11 players if Pep was no longer coaching City at this time, and I think that makes perfect sense because Pep is a real key player at City.
Pep succeeded in building a solid squad, which was able to work together as a team, and was able to manage the egos of its players to bring out their best potential. That's the basis for Pep success in overcoming the challenges before them. Indeed, there are one or two players who stand out, such as De Bruyne, Halaand, Silva and Rodri, which is indeed beneficial for them, but this does not negate the contribution of other players in the team.
Man City will face Copenhagen in the round of 16, I am sure they will be the winners in that match. Because I think Copenhagen qualified for the last 16 because of luck, where Man United performed badly in the group phase. So it was an easy game for Pep and his fleet, and about the chance of a surprise, I think Copenhagen is having a hard time making it happen, they need a big miracle.