Well, if De Bruyne can't play it wouldn't change much on the side of the citizens, because pep guardiola always has a replacement. However De Bruyne plays a pivotal role on that attacking midfield, because he has the ability to distribute the ball and sometimes make a goal attempt. Pep hasn't built Manchester city on a single player, so arsenal will still have a tough game on Sunday. The tactical manoeuvre pep guardiola normally uses is to conquer the midfield and force the attack, which normally doesn't work much against arsenal because they too have players saturated at the midfield with solid defensive midfielders as well.
But maybe the injury is not too serious and he can play during the match because from the various media that I searched, no one said that he had a serious injury and I hope that is true. De Bruyne's presence is quite important in Pep Guardiola's squad because he is the organizer of attacks and the one who neutralizes the midfield to make it more lively. Without De Bruyne they still have another midfielder who can make the game livelier but it is a little different to have in the team.
Arsenal's weakness is in defense and I see it will be quite difficult for them to match Man City's attacks. Like when the first leg took place, even though at that time De Bruyne couldn't play due to injury. But Arsenal was lucky enough to win the match because they managed to score a goal in the last minute before the match ended.