With the signing of De Ligt and even though Bayern Munich lost Lewandowski. It feels like they are a stronger team now than they were 2 months ago. They needed better defensive players and they've now got De Ligt with a 5 year contract. In terms of attack, they've got Mane to run down the wings and create more chances.
Bayern Munich's defense and midfield are strong enough for next season. But with the loss of Lewandowski, I'd say Bayern Munich's attack will be a bit weaker. Sadio Mane cannot be considered as Lewandowski's replacement. In the last 3 seasons in the Bundesliga, Lewandowski played 94 matches and scored 110 goals. Sadio Mane doesn't seem to be able to score that many goals. So if Bayern Munich does not get a new striker, then they have to change their match strategy.
After Lewandowski leave i think people shouldn't too worried about forward side for Bayern because they hasn't lack of strikers and besides Sadio Mane they have Thomas Muller and Kingsley Coman who can playing as striker although their records not too fantastic such as Lewandowski but they were playing good too last season but i interested one name on Bayern who have striker position too and he is Joshua Zirkzee that after loaned by Anderlecht last season he can scoring 16 goals on Belgium league so as 19 year old player i think he is very potential to being an future striker for Bayern and if he develop with very well i think he can be considers as the successor of Lewandowski