It is so bad that Dortmund do not even care if selling their players will make the team to be in a bad form, they just keep on selling their players just to generate funds. If Dortmund was never interested in selling their players Dortmund would have been a great team not just in Bundesliga but also in the Champions league. Bellingham is a great player that would have been very helpful for Dortmund but they do not think about keeping players to make the performance of the team good
Dortmund has face difficulties this year particularly after losing Bellingham and other important players. For their long-term success it is essential to develop new skills. Unquestionably the UCL preliminary round is under pressure but Dortmund has a reputation for perseverance. I think even if the group that includes PSG, Milan and Newcastle is challenging, unexpected things may happen in sports. Although they have benefited from the weak competition in the Bundesliga Dortmund supporters are hoping for a change
But in reality, retaining great players also requires large costs, because when a player has greatly improved performance then of course the player will also ask for an increased salary. Therefore, it is not surprising that Dortmund will always sell the great players it has, because apart from business and big profits, at least Dortmund is also not ready to increase their salaries to keep them from leaving. Anyway, cases like this are nothing new and also, you don't need to worry about Dortmund performance because over time Dortmund will certainly recover.
I have no doubt that Dortmund's performance will improve. Because we have been seeing the same thing for the last few seasons. They sell the best player in their squad every season. And then their performance is unstable for some time. But with time they are able to recover themselves. This season they sold their best player to Bellingham. And now their performance is somewhat erratic. In three matches, Dortmund won one and drew two. If Dortmund's star striker Haller is able to perform well, maybe the consistency of Dortmund's performance will return.