Erling Haaland is most important part for Manchester City and has small opportunity without him will get highest phase in this season, if can't recovery as soon possible I doubt with their opportunity will lead until final round for this season or not.
Haaland is not the most important player in Manchester City's team, he also contributes to the team's success in the last Champions League victory; If the team have him, it will be an added advantage but without him, they will still play well since De Bruyne is back from Injury.
Yes, having Haaland will increase the attacking power for the better, but if we flashback to last season when in the semi-finals and final of the UCL he couldn't do anything because his movements were always broken by Madrid and Inter Milan's back line, actually I don't think so either. Haaland can do a lot in the UCL this season, even though he has always been Guardiola's mainstay in the front line. KDB's return is actually more than enough.
Without Haaland, it seems that Manchester City will be fine in facing their opponents in the UCL. I think they will be successful again this season in reaching the final because Manchester City has risen from adversity since KDB managed to recover from injury. I also hope that Manchester City will win the title again this season. this is as perfect as they did last season, no matter without Haaland I think with the main squad which is currently more than enough in my view.
The round of 16 will soon start where they will face Copenhagen who managed to become Runner UP in group A which is quite difficult but they managed to escape with a surprise, I think Manchester City cannot underestimate them, even so they could make things difficult for Manchester City later.