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.
Yes, that's right. Barca's financial condition is indeed very worrying and this is very different from its rivals who have stable finances. So this season Barca can't do much to compete in La Liga with Real Madrid, because Real Madrid's squad is better even though Real Madrid started the pre-season badly after losing to AC Milan but that doesn't affect that Real Madrid is a strong team this season.
If Barca wants to return to its former glory, Barca must focus on improving its financial condition first, looking for talented young players who are not too expensive. Because as I recall, Xavi was also fired for criticizing Barca's finances too much. So I conclude that Barca's real problem is finance, if this is resolved Barca will be successful again because they can bring in great players and no less importantly the condition of the dressing room will also be good, there will be no misunderstandings between the coach and the players like what happened to Xavi last season. Hansi Flick's job is not easy, so he needs time to return Barca to being a team that is feared by his opponents.