the possibility is injury. the hope is that he will be in the squad because indeed without De Bruyne's presence Manchester City have lost a good supply of bait in midfield even though they are still very good but with De Bruyne's presence they are clearly much better.
It doesn't seem like an injury, because after I had a bit to find out it looks like or maybe Kevin De Bruyne is sick but not injured. Because when Manchester City played in the Champions League against Leipzig, in fact Kevin De Bruyne was not included in the squad.
But after that, when Manchester City played in the Premier League against Bournemouth, the reality was that Kevin De Bruyne was on the bench. So yes, Kevin De Bruyne is not injured but only has an illness and hopefully Kevin De Bruyne will get well soon so he can play in the second leg against Leipzig because Kevin De Bruyne's presence in Manchester City's main squad is still very much needed.
Even though their wings are still very good, Manchester City is incomplete without the figure of De Bruyne in the midfield.
Even without many players I firmly believe that Manchester City will qualify further because if we compare both teams players individually then we realize that even with bench players Manchester City is clearly better than Leipzig.
The fact that they are playing at their home field in front of their fans this second leg is another beneficial factor for them and most likely Leipzig will suffer a defeat with a big number of goals scored,Manchester City focus is let's say a bit more in the Champions than the Premier League right now.
I agree because from what I saw in Manchester City when they are on the field they are not that much dependent on their players for any team midfielders are very important for both playmaking and defending and there is the same story with Manchester City while they can even use another player instead of Kevin De Bruyne or change the tactic but generally, a team shouldn't be depended to a single player doesn't really matter how he helps the team.