It's still unclear whether Sadio Mane will replace Lewandowski but one or two Bayern players if they don't have a big impact because the Bayern team is already very solid on all fronts.
Sadio Mane has reportedly agreed to join Bayern Munich next season. But for Lewandowski there has been no recent news regarding him whether he is still staying at Bayern Munich or actually leaving. If Lewandowski still survives it looks like it will be a better collaboration than Mane and Lewandowski in attack.
We have not yet seen any official news about Lewandowski's departure from Bayern Munich. I heard rumors that Lewandowski was not interested in staying at Bayern Munich. And Barcelona wants to take Lewandowski into the team. But Barcelona is still in financial crisis. In this situation, it is very difficult for them to get Lewandowski on the team. But if Sadio Mane and Lewandowski are in Bayern Munich's forward next season, Bayern Munich will be even stronger. And they can expect better results in the Champions League.
I believe that this transfer signals the end of Lewandowski time at Bayern, and
Barcelona now have the funds so they’ll buy him in coming days. Furthermore if Mane doesn’t have a big impact immediately then Bayern will have a bad season, because replacing Lewandowski was never going to be easy and Mane is coming from a different league hence Bayern will be hoping that their gamble pays off.