We can see Manchester City's performance in the Premier League without De Bruyne and it is clear that the strength of this team has slightly decreased, whether you realize it or not, now Manchester City seems to have lost its impressive style of play like last season, so far Man City is still getting positive results without presence of De Bruyne but I don't think it will be when they play a stronger side in UCL.
surely losing De Bruyne will have a huge impact on the Man City team which will reduce the strength of this team.
Even though playing without De Bruyne, Performance might have decreased a little, but apart from that it also seems that Manchester City is capable of winning consecutive matches to make Manchester City top of the EPL standings, I know that De Bruyne's role is very important in the main Manchester City squad, but It seems that Guardiola knows how to place other players to temporarily replace De Bruyne to make Manchester City's attack power not dull.
Guardiola, of course, always prepares other strategies if a key player in Manchester City is injured, I'm sure he's already thought about that and just look at it, it seems that Manchester City without De Bruyne can still play well and their attack power is still very good and Haaland isn't infertile anymore, even him often scored goals for Manchester City's victory. I'm sure that without De Bruyne, Manchester City can still play well in the UCL later.