Kevin De Bruyne is one of the most valuable players for Manchester City without doubt. His absence due to injury is really unlucky for them as well. However I don't think this would affect them deeply anyway. Because they are still one of the strongest teams in Europe now. They are full of world stars playing in coherence together.
There has been some drop in their form recently but it is no problem at all. I remember Manchester City having way worse times in one season and they were way behind the leader. But they still made an unforgettable comeback and won the title by a serious point gap.
There is no Kevin De Bruyne in the squad will have a big impact and so far Manchester City have suffered consecutive defeats in several previous matches without the best midfielder that Belgium has. They are still the strongest team in Europe and the problem is that they cannot achieve a promising victory without the presence of an energetic midfielder and losing several other players due to injury could also result in Manchester City losing.
The tight competition schedule has caused several players to experience fatigue and injuries, this condition will be quite dangerous for Manchester City and this condition cannot be attributed to last season because currently several clubs in the Premier League have shown big changes. So the competition will be much tighter and especially they play in the Champions league.