First of all, they lost Robert Lewandowski. Then after that, Choupo Moting seems to be playing quite decently. Even though he was not able to fill the void that was left by Robert Lewandowski, but he was at least trying to do what his team needed him to do. I do not know if Füllkrug will be able to produce what even Choupo Moting was producing. This has been a very sad situation for Bayern Munich. They wanted to improve their attacking capabilities by bringing in a competent player with the money that they got by selling Robert Lewandowski. But that plan absolutely failed.
With the money that Bavaria received from the sale of Lewandowski, it is impossible to buy anyone at all who would correspond to the level of Bavaria. But Bavaria has no shortage of money and can afford much more expensive purchases and I hope they accept this reality that the "cheap" options have run out. I think that Füllkrug is just a cheap option that doesn't fix anything. If you make a purchase, then it must have some meaning.
Bayern Munich is very careful in spending their money, they will not spend money on players of standard quality.
Bayern Munich is not a club that is in financial trouble, the sale of Lewandowski is not much with the cash they currently have. If they bring in players for short-term targets as a coating option for injured key players, they will choose free transfer players. Reportedly Bayern Munich are starting to be interested in signing Ronaldo after his status as a free transfer player, Bayern Munich does not need to spend money to sign Ronaldo as an option to replace the injured Mane.