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.
First they lost Lewandowski, second they got Mane to replace Lewandowski. Third, they lost Mane because of injury. Last of all, they luckily got a very good contribution from Choupo-Moting. Even so his performance in World Cup proves that Choupo-Moting is not a world class striker and his performance is benefited by a good system in Bayern.
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.
Bayern are waiting for recovery of Mane but they will have to find another striker to replace both Mane and Choupo-Moting. They are old players already and to win big titles, Bayern will need to have world class strikers.
In their club history, they had many great strikers, Gerd Muller, Elber and Lewandowski is their latest great striker.