Haaland does have a huge impact on Dortmund's performance as he's scoring a large percentage of their goals and there is no one denying the ability of him in terms of scoring goals.
But I have to say that when Haaland was injured, Dortmund didn't have a huge lot of problems in the scoring department. Maybe they struggled in a few matches but they overall did okay without him.
To understand the importance of an individual player, you need a distance greater than a few matches, football is a team sport, so the individual contribution of even a star player is very difficult to evaluate. For example, when Ronaldo left Juventus, the team found itself in a deep crisis and this manifested itself very quickly, although formally there were two more very good strikers in the team - Dybala and Morata. Haaland is very important for Borussia and they will have to look for an adequate replacement for him if he leaves.