Lewandowski was able to answer Xavi's wish by scoring goals in his first season with Barcelona. It is undeniable that Xavi must look for Lewandowski's cover because Ansu Fati, Ferran Torres, Raphinha, Dembele are not pure centre-forwards.
Barcelona's ideal 4-3-3 formation is slightly advantageous if Xavi defends with one centre-forward as a winger can be a goalscorer assisted by a creative midfielder but there are times when Lewandowski gets injured and will turn into a weakness. At least that's what I feel.
Barcelona decision to sell Aubameyang and then replace it with Lewandowski was very correct, Lewandowski was able to answer Xavi trust in him by helping Barcelona win the La Liga title race as well as the Super Cup. Lewandowski was also the top scorer last season which made Benzema fail to win the top scorer title twice in a row.
Xavi can still rely on Ansu Fati as a substitute for Lewandowski if there are problems with his health. Raphinha is starting to blend into Barcelona style of play, he can be relied on on the right wing. Lewandowski certainly cannot always be played in every match, especially since Barcelona match schedule this season is very tight, Xavi needs to rotate in several matches in order to maintain the fitness of his players while preventing injury to his flagship player.
That's what I feel is why Xavi needs to look for a pure striker like Lewandoswki. Ansu Fati is not a center forward and Fati's real position is as a winger. It will not be optimal to make it an option to cover Lewandowski's position.
After getting Gundongan from Manchester City, Barcelona signed Vitor Roque from Athlico Paranaense on July 5, who plays as ST. IMO, Vitor Roque's posture is also not much different from Ansu Fati and will experience difficulties when dealing with opposing defenders who have a larger stature.
I'm trying to repeat history when Barcelona signed Thierry Henry and Ibrahimovic. Barcelona's goal is to bring in the two players I mentioned to win the aerial duel even though the results were not as desired. But it's important to consider.