Kevin De Bruyne in the best shape possible being absent makes the team worse. I don't wanna come up with a % number by how much the team is worse, but what would the point be of being proud to have this exceptional player when he does not give the team some % points in extra performance? No, for me he is one of the best midifielders/midfield-wingers of all times. There is hardly any player who understood as well how to put his teammates into the perfect position to score. His assist numbers are widely unmatched. I don't know whether there is a leaderboard regarding number of assists historically, but if there is, I am sure he is to be found in the top 5 of all times. I am not sure how it would not be a loss for a team if such a player is absent.
Yes, Kevin De Bruyne is a very influential figure for Manchester City, he is a great leader whose presence in the team will certainly have a tremendous impact. This season I think Manchester City's weakness lies in the midfield if De Bruyne cannot play. This season, Pep Guardiola is certainly very aware of this because he lost two important figures, De Bruyne, who often suffered injuries and lost other important figures such as Gundogan, who chose to move to Barcelona. The entry of Kovacic has not been able to be a suitable replacement, so Pep Guardiola has to change his playing pattern very often, which has caused Haaland to be a little hampered up front. But City's chances are big enough to get full points because playing at the Etihad Stadium, the support of fans could be very helpful in suppressing the opponent's mentality.
Everything you said is correct and many people forget about Gundogan and how he built the most dangerous midfield in the world with De Bruyne last season. These two guys had countless of goals and assists to their names and they perfectly harmonized with each other when it came to controlling the speed of the game. The fact that they scored so many important goals was another big advantage that Manchester City had over other teams. De Bruyne scored the important goal in the first leg against Real Madrid and Gundogan scored several important goals in Premier League games, some of which he scored late in the game when the strikers couldn't get the job done. Having such a midfield is so valuable to any team. It means that not everything has to be done and managed by a player like Haaland. Instead, the whole team is dangerous and can score goals. It is a bit different this season, but I think that was to be expected as it is always close to impossible to replace such a midfield 1:1 within just one season's time.