That's exactly the point. Kevin De Bruyne is a key player in Manchester City and currently, he is even more than just a key player in this team because Kevin De Bruyne's performance in Manchester City makes this team win or lose a match and if they lose Kevin De Bruyne, they can't have good performance anymore. you just saw in the games when they didn't have Kevin De Bruyne because he got injured, Manchester City had a bad situation in the Premier League and I think if they don't have Kevin De Bruyne, Manchester City will easily fail the Champions League.
The quality of Manchester City's game without De Bruyne is very different from the game with De Bruyne. He has proven to be the motor of Manchester City's attacks, his presence on the pitch makes it easier for Manchester City to score goals and win. I am very sure that De Bruyne's presence will really help Manchester City in defending its title, he will continue to give his best performances to make Manchester City ahead of its competitors again.
His presence will make other players more enthusiastic in controlling the match and organizing attacks, he is also not only smart in providing assists for Manchester City strikers, but can also be relied on to score goals that make it easier for Manchester City to secure victory.