Indeed City played a great game but they need De Bruyne back as quick as they can if they want to go back 2 back UCL champions. This season is way hard for them to conquer the trophy and from my point of view , I do think Bayern are the 1st favorites and then City and Real but you are right , De Bruyne presence will be felt as soon as he steps on the pitch and City also needs him for EPL fight. 
Manchester City really needs experienced players to achieve their target this season to become UCL champions. Well, Kevin Bruyne is a very experienced player and he is the team captain, of course he is quite needed for every match that Manchester City will face. In fact, Kevin return to the Manchester City squad in today match was welcomed by fans because he had been absent for quite a long time while recovering from his injury.In my opinion, Manchester City is quite strong this season, indeed without Kevin De Bruyne in the group phase, it can be covered by the good performance of other players. But Kevin return to the team is of course welcomed only by Manchester City players because he is the field controller that Manchester City really needs. Maybe Haaland has found the best bait supplier for himself in the upcoming match
You explain this as if Manchester City was lacking experienced player, but I don't think they are. Most of the players they now play with also were the players who won the Champions League trophy. The small bad run they are having is not about a lack in experience within the team. It is more of a basic thing that important players are missing and some changes left some marks obviously. When important players leave the club that initially contributed a lot to everything they achieved, it is normal that the club will experience difficult time and a new episode within their club history. I think they will play a better second half of the season.