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.
Manchester City performance has been affected by the absence of KDB in the squad as it has been in their games despite having a very solid team one of the best team in the world but their position in the EPL and their game play says it all because the impact of KDB has been lacking in the team. Manchester City has managed to get to the round of 16 in the UCL I think they can still struggle to win games in the UCL even without the presence of KDB but the team will be in better shape and there will be more distribution of passes and more chances will be created if KDB should return from injury, hopefully he's going to be in the squad soon because City performance in the EPL has dropped so much.