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.
Last season, Man City actually experienced a fairly bad decline in performance. But it does not last long. They only slumped slightly in the Premier League when they were more focused on the Champions League which is currently at the semi-final level and heading towards the Final. So at that time Man City in the Premier League experienced several defeats and draws. But when they managed to produce good results in the Champions League, Man City proved again that they would also be stronger again in the Premier League. So last season Man City won 3 competitions at once, namely the FA Cup, Premier League and Champions League.
Maybe this time nothing will be different. Injuries to several players have indeed caused Man City to experience a slight decline, but this is definitely only temporary and when the players recover, Man City will return to their best performance. But if injuries attack other core players again, perhaps Man City won't get many trophies this season.