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.
Indeed, Lewandowski is already old and cannot guarantee that he will remain fit until the end of the season, therefore Barcelona desperately needs a backup strategy. On the other hand, Ansu Fati is a winger like Vinicius, and I don't believe he can play as a striker. He has good finishing ability, primarily from the wing, but it might be challenging for him to play forward because of his lack of experience. Instead, Xavi ought to seek out a different striker who is a natural no. 9, even if that player is from the fielder team.
Ansu Fati is like a forward than a winger but he has to show his ability more if he is too slow, he will be sold out. Barcelona can not wait for Fati too long as three seasons since his debut for Barcelona in 2019 -2020 season, he has been given some years to grow up.
Ansu Fati profile and there is rumor that Barcelona want to sell him to get money and it seems time is counting down for him. A coming season is important for Ansu Fati future in Barcelona and he should not waste that chance.
We all know how dangerous Ansu Fati once was, but after suffering a career-threatening injury, he substantially decreased in speed and ability, so I can't really blame him for being slow. He certainly couldn't have handled the injury as a young player, and Barcelona is lucky he was able to fight his way back. He is playing cautiously and is working hard to go back to his best, but it will be challenging because the doctor warned him to be careful and that another injury would mean the end of his career.