Manchester City until now played without De Bruyne and they can still achieve 3rd position in Premier League and won 2 matches in Champions League. They're already been great so far, even though De Bruyne is currently a Manchester City's heart.
De Bruyne will also get older, it's just a part of regeneration and Manchester City seems quite success since they're still in good form.
Manchester City still on the top performed in Champion League by winning 2 of 2 matches without Kevin De Bruyne but they have bad result after losing two matches in a row at Premier League. I don't see success achievement for Manchester City without De Bruyne although have many young player and need time for Pep Guardiola adapting new games play without De Bruyne. Current group stage opponent teams in Champion League looks not problem for Manchester City keep in the winning trend, but I doubt on knockout round if Kevin De Bruyne not recovery from injury will Manchester City have top perform or difficult qualifying until final stage.
Difficult to see better tactical with Manchester City actually Kevin De Bruyne have been vital player and helped Erling Haaland easily scoring many goals and looks difficult scoring goals when De Bruyne absent.
Well, Kevin De Bruyne is a very important player in the midfield for Manchester City. It is nearly impossible to actually find a player like that. So of course when he is not there, Manchester City is going to find it really hard to manoeuvered the ball in the midfield area.
Does that mean Manchester City is useless right now? No absolutely not. But the freedom and authority that Manchester City had over the midfield in almost every match is not there right now without him. And without him, the supply for the forwards are also not going to be amazing, to be honest.