Dortmund needs to look for a replacement for Haaland because it is very likely that Raiola will find him a new club this winter. Of course one should not forget about the defense because indeed in Dortmund there have been quite big problems with it lately.
It would be really good for Dortmund to start doing some research about some strikers already now. Because Erling Haaland is very likely to leave the team in next summer. I don't believe that some club can manage to sign him in January already but I'm very sure about the summer transfer window. Clubs like Real Madrid, Manchester City, PSG and Manchester United are already thinking seriously of signing him. If Dortmund can't fill his place with a solid striker, then they can have bigger problems in the next season I assume.
There is a good chance that Haaland will leave. The question is not if he will leave, but when. A player of his caliber is not easy to catch. Or maybe they could make a deal with Madrid that they get Hazard and then some for Haaland, as Hazard is currently experiencing the worst period of his career. There must be the financial possibilities, something like that could be an option to absorb it.