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.
Barcelona don't have the luxury of buying as many players as they want. Unlike Real Madrid, who can buy any player they want, Barca's financial problems mean they have a spending limit. Therefore, Barca need to build a very balanced squad. Therefore, for Hansi Flick to be successful, he will need to keep players like Vitor Roque, Eric Garcia and he will need to know how to get the most out of every player at the club. For now, Hansi Flick's job is not easy and Xavi said before leaving Barca that Flick must be ready to suffer.