It seems like Borussia Dortmund are also one of the teams that are after Manchester United striker Anthony Martial. If Erling Haaland leaves the club at the end of the summer. They will need to fill his place. But I don't think that Martial is the right forward to fill the place in. They should look for much more capable strikers. Otherwise, they will be regretting that they sold Haaland.
If Haaland transfer really materializes, of course, Borussia Dortmund should be able to move quickly to find a replacement, but just like you, I also don't believe that Martial will be able to replace Haalaand, so far Martial performance at Manchester United is really bad and from season to season the performance martial continues to decline.
They already have some young capable strikers like Moukoko and Malen who was recently signed and already have scored some goals for black and yellow so I think getting Martial would be decent. It would be much more fruitful for United though as they would able to swap an out of form striker for a next world class striker. It wouldn't be easy for United as well as they would be asking for lot of money to let him leave to United.
So far it is very rare for borussia dortmund to bring in players who have seniors to replace their attackers who leave, i think dortmund may be orbiting one of the talented young players to be like haalaand, dortmund is a club that every season must be willing to lose players star, but instead they continue to be able to get other young players who are more talented.