The first thing that comes to my mind is that Robert Lewandowski was a central striker, and Sadio Mané is a winger, and you can never replace a striker with a winger.
There is no doubt that Sadio Mané has scored quite a few goals, to be precise, he has scored 9 goals. However, matching the tally of Lewandowski is going to be really hard because a few seasons back that guy scored a whopping 41 goals in one season, which was the highest ever goal scored in the Bundesliga.
I really think that they should bring in a new striker in the next transfer window, otherwise, they will suffer quite a lot in the champions league if they don't do that.
In the case of Sadio Mane, Nagelsmann hopes Mane can fill the position left by Lowandowski. that doesn't mean that wingers can't be center forwards. it's just that, in the case of Sadio Mane, he doesn't fit the position. moreover, in the style of Ngalesmann's strategy of frequently tweaking the positions of his players. until in the end, Mane took his original position as a winger. and it seems, he is more comfortable.
Bayern still have a centre-forward option, although Choupo-Moting is not as sharp as Lewandowski. however, Bayern Munich is currently a very productive team. in 15 matches in the bundesliga, they have scored 49 goals. something extraordinary, even without Lewandowski. I believe Bayern Munich's management have planned to bring in newcomers this January or the next summer transfer window. at least, there are already several players who have always been linked with Bayern.