Hansi Flick will have difficulty to improve Barcelona defense if he wants to gain success with Barcelona. If he fails to improve defense system for Barcelona, there is very small chance of success there. Furthermore, attacking power of Barcelona was affected a lot last season with decreasing performance of Lewandowski because of ageing. Ageing effect on Robert Lewandowski can become bigger in a new season and this bring another big difficulty to Hansi Flick.
I can't easily conclude on Hansi Flick's performances in Barcelona because the season has not yet started, but what I can only say now is that with this current Barcelona squad, it will be very difficult for Barcelona to fully compete with Real Madrid in the La Liga trophy run. Additionally, in other tough competitions like the Champions League, it may be very difficult for them to compete with other top teams.
Barcelona's squad is not that bad, but they need some quality players to strengthen the team's performances so they can compete with other teams in the Champions League. With a tactically adept coach, even average players can be turned into high-quality players. We saw what Girona achieved last season; despite not winning any trophies, we witnessed their great performance. If Girona could play well last season with an average squad, it is very possible that Barcelona can perform even better with a good coach.