The last manager of Barcelona before Flick is Xavi and during his time with Barcelona he created a good impact in the team which is part of what the new manager is enjoying now because Xavi brought the players that has helped the Barcelona team to win the league last season. Flick took over from Xavi to continue where he started but the tactics of Flick has helped the Barcelona team to regain control of the league so I can say Flick is the better manager than Xavi.
Flick has been doing so well ever since he took over from xavi, the legacy of xavi did set a very good foundation for Flick to build upon and he has been doing so well with it all this time. Barcelona has done really well and has achieved much having flick as their manager but this should not be good reason to compare both of them because they managed the team at different times when the team had different kind of challenges they faced, so they have both done well and not have any as better. They are good at their times
Not trying to give the glory to Flick for what he has achieved so far since he took over as the Barcelona coach but looking at the players that was on the Barcelona coach during the time of Xavi I don't see much difference between the current players now and then because Barcelona is still in financial crisis but the current manager is managing the team and he has used same players to achieve a great result in the team.