Its not really bad performance from Sadio Mane last season with Bayern Munich during 25 games scored 7 goals and has 5 assists, exactly he was injured and absent for many games make him difficult on the top performance. Sadio Mane have huge pressure exactly arrived there for replacing Robert Lewandowski consistent become top scorer in Bundesliga. But not fair with comparison between Sadio Mane and Robert Lewandowski because Mane is not as real central forward and he most likely play as winger position.
I think Sadio Mane can back to the top perform next season if Tuchel put him on left winger side than push as center forward, Mane will create many assists like did with Liverpool and how his contribution make Firmino and Salah scoring many goals. Need change with Mane position and Bayern have to sign or find new center forward next season.
Yes, I agree with you that the injury factor has reduced Saido Mane's performance, statistically he is also not a pure striker with number 9 like Lewandowski. Despite his temperament and tension with his colleague Leroy Sane Tuchel knows very well what Mane's quality really is because he often faced him when he was still coaching Chelsea.
We can also see how the Munich transfer market is moving this time, who want a centre-forward, namely Hary Kane, this shows the fact that they have no problem with Mane but they don't have a pure striker like Lewandowski.