While I do agree that Haaland scores a lot of goals and he is a key figure for Manchester City, lets remember that he has been there for two seasons, this is his third if I am not wrong, and City won last 5 titles out of six, with four of them coming back to back. So that means they were good before him as well, he is not the reason why they win, so if they won without him, I am sure that they will do that again.
Not like if Haaland gets injured, that means they have to play with ten players, they will just put someone else in, and they will win like how they used to win. This isn't a simple thing, and I agree that City would hurt a lot, like how KDB was missing last year and they had trouble, but they didn't become terrible, they were ok enough compared to everyone else. Haaland scores a lot on unimportant games too, not like he scores on UCL knock out stages that much, so what they are missing would be some player who will score 4 goals against league last.
The contribution of one of these attackers is very influential in his team, indeed Haaland is the key to Manchester City because he has his own advantages with his good performance also supported by his young age which of course makes him still have full stamina to compete. The title victory that they got in a row is a very good thing, besides that with the long-buried rumor saying that Pep Guardiola will leave Manchester City and if that happens then he said Haaland will also leave his team, if this really happens then it will affect Manchester City's performance.
Of course if Haaland is injured I think it could affect their performance, although not big but their coach must find a replacement who can replace this position for a while.