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.
I'm not sure the strength you are talking about but Manchester City right now are the only team player with 3 straight win matches with the equivalent of 9 points which as a result places them at the top of the standing, in addition, they have only conceded only 1 goal of the 3 matches they have played with a total goal of 6, that is fair if you ask me and to see that Haaland has 3 goals in the three appearances is excellence, still among the top players that are competing to be top scorers because right now, there 4 players with 3 goals score but the coming weeks, we will see changes as Haaland will definitely score another goal against Fulham in their next match.
Kelvin De Bruyne's absence was felt but the team is still doing fine, I'm happy to see that Haaland came second in the UEFA Men's Player of the Year after Lionel Messi and De Bruyne came third, the two players from Manchester City are making the Etihad proud, waiting to see him come back and defend his club and see what he is going to add to Manchester City this season.