Their midfield was key last season and if we now think about who is left of that midfield, both are gone. Gundogan and De Bruyne. Since the two controlled the whole Manchester City game and also scored goals and gave countless assists, it doesn't come as a surprise that now when both of them are absent the game is much worse than it was last season. I think this is probably the difficult task ever for Guardiola to find two successors for Gundogan and De Bruyne who are able to manage the Manchester City game on the same level.
De Bruyne is about to be back after a bad injury time period. He is one of the most important players for Manchester City now clearly. His creativity and great skills at finishing as well make this team much stronger in big matches as well. Haaland - De Bruyne has made a great duo so far as you know and Guardiola would like to see both of these players back as soon as possible.
Haaland's return might take some more time but for now it looks like he might be back in one or two weeks the most. Gundogan was also an important part of the team before as you said but I think Manchester City are still doing okay without him in the midfield. De Bruyne's return will secure the area more.
Manchester City benefits greatly from De Bruyne's return. The man is creative and skilled. It's clear they miss him and Haaland. Their absence has cost City points, clearly. De Bruyne's return will boost the team, especially in big games. I'm doubtful he'll recover quickly. Long-term injuries can be physically and mentally taxing. For Haaland, patience is crucial. I know everyone wants him back, but health comes first. Gundogan has played well, but De Bruyne's return will give City's midfield that extra spark