If Kevin De Bruyne, Jhon Stones, Bernardo Silva have to be sidelined until September, the Man City squad will automatically not be strengthened by these three players in the Champions League Group Phase. I think the only option now for Pep Guardiola is to add to his squad in the remaining transfer window, Guardiloa said that the Club will take an important decision to deal with the current situation. I think this is an indication that City will recruit new players, and of course they will find their target players easy.
I predict Man City without the presence of these three players in the UCL will be a little different, because the competition demands experienced players and has a winning mentality like them.
Ok but the Manchester City style is very challenging and players usually need quite some time to perfectly adapt to it. I remember when they bought Grealish for so much money and everyone expected him to be perfect from day 1, but that didn't happen. He really needed time to find his optimal performance level and also spend some time sitting on the bench at first. That's why transfers for crucial positions in such a team is not easily done and I wonder which players are available at all.