Hansi Flick proved himself last season, so there's no doubt about his performance this season. However, I think the numerous injuries will have a slight impact on the team's performance, even though there are adequate substitutes. I think things will be a little different.
This season, Barcelona is riding high on confidence after winning the title. What happened at the start of the season can't be used as a benchmark. I'm still quite optimistic that Hansi Flick will be able to bring Barcelona success again this season, possibly even better than last.
Flick did well last season no doubt but that doesn't mean this season will be the same as last season. Injuries might not just have a minor impact on the team, it can change the whole rhythm of the team especially when it affects key players like Pedri, Yamal and Raphina.
Every season is different from the previous. Other teams have studied Barcelona style of play, improved their squads and will be more improved to face Barcelona. So using last season to judge can be a mistake because football is about what you do now, not what you achieved last season.
You are right that Barca performance last season may not be consistent this season. Adding Marcus Rashford to strengthen the team was a great decision for them but he has not made any significant contribution to the team yet. Yamal and Raphinha have made more contributions and they are trying to play as well as last season. But Hansi Flick should be more aware of the poor performance against Rayo Vallecano because the players need to train hard to improve their abilities.