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.