Even without Kevin De Bruyne in Manchester City squad, they will qualify to the round of sixteen because the team is not a bad team that solely depend on only one player. I will agree with everyone that without Kevin De Bruyne, their performance is not as last season and they lose some matches they are supposed to win due to his absence, and I will also agree with you if you said that he is a player that if Manchester City play without him, they might not win the Premier League as well as Champions League.
Manchester City is already in the last sixteen now in the Champions League and in the middle of next month all teams including Manchester City will compete again to reach the last eight. However, without Kevin De Bruyne in the Manchester City squad this season, of course there will be a little difficulty for Manchester City to reach several important rounds in the Champions League. Because other teams have also improved rapidly in their respective strengths, Manchester City will not run smoothly in the two big competitions this season.