Overall, Barca is not weak, but the important factor to end the match is the attack. They are lacking a really sharp striker. They can play a dominant style of play well, holding the ball a lot thanks to a pretty good midfield, but if they don't score, they can't finish the match. Therefore, I think it is difficult for Barca to go far in the upcoming UCL.
Indeed, Barcelona's problem is in the attack line and that is why many parties doubt they can qualify for the next round. Moreover, the opponent was the defending champion of Serie A last season, namely Napoli, but Napoli themselves were also in poor performance. Maybe for Barcelona's attack line they can temporarily entrust this role to Ferran Torres, especially since this player has just recorded a hat trick in the previous domestic league match. Barcelona's new striker who just arrived this winter, Vitor Roque, is also not very convincing and is not the same as he displayed at the previous club.
Maybe we can't expect much more from Barcelona and if they manage to overcome their shortcomings in attack before the first leg starts then that will be quite promising. It's not easy and they have to do extra rigorous training so that the target can be achieved and Xavi has to be correct in responding to this.