Lewandowski is a main goal source for Barcelona but they have many attacking players and it's impressive that they have many players already scored goals for them. The list of scorers for Barcelona is long and shows that they have different attacking solutions for finding goals with many aces under sleeves. Lewandowski is playing well but he is old and his performance quality will probably drop in second half of season.
Hence, Barcelona need other players to cover this issue for Lewandowski and they will win La Liga trophy if they succeed to do this task. So far they have been doing this task very well and their young players will show their bigger contributions in this season, when they become more experience by playing more matches.
Yes, Robert Lewandowski has been Barcelona's key player so far this season, including the impressive 4-0 win over Real Madrid. Lewandowski's form was sometimes criticized last season, but he has been excellent under Hansi Flick. I won't overstate how Lewandowski has led the youth revolution at Barcelona. The explosive form of Yamal, Raphinha, and even Dani Olmo is also thanks to Lewandowski's guidance. I was impressed by Dani Olmo who scored four goals in five La Liga appearances for Barcelona. To achieve this, the Spaniard only had six shots on target. Yamal became the youngest player to score in an El Clasico, while Raphinha continued his rise after scoring a hat-trick against Bayern Munich in the Champions League.
Barcelona still really needs Lewandowski, he will guide the younger generation to grow up, at this form he can still play for Barcelona for another 1-2 seasons. There is no need for Barcelona to find someone to replace him.