Kevin De Bruyne and Haaland were two important figures in the Manchester City squad last season, so when one of them suffered an injury and was absent from defending the team, it certainly disrupted the team's performance, especially if both of them were not there.
However, if we talk about the possibility of winning the Premier League, I think Man City is still very much in the cards and with quite a lot of games remaining in this season, anything is still possible, especially as Manchester City is currently only 5 points behind Arsenal who are at the top of the standings Premier League.
You forgot to mention Rodri in the list. With the absence of De Bruyne, Rodri’s presence has been very important to the squad because whenever he got injured, the team always feel it, likewise whenever Haalandbis in the field, they have that confidence of winning even though if they are without ball he is an irrelevant player, but if they created any chance he will show his quality as a good striker that is why they missed him due to injury.