Flick took a gamble and he's lucky it worked out for him because in the first half, aside from the goal, Ferran Torres who replaced Lewandoski in the starting 11 was very horrible. Everything about him was off. His first touch was awful, his runs, his link-up play, positioning, everything was just bad. If they had a clinical striker up front, the score could easily be 2-0 in the first half.
Because there is no way for them other than trying to continue with their youth especially for young players to be able to play stable when Barcelona needs to rotate players and focus on several competitions at once.
Flick is currently trying harder to prove that he deserves to replace Xavi as coach and of course the initial achievements are important where they are definitely pursuing the target of several trophies at once as a plan for this season, especially for the focus is the Champions League which is indeed in the Xavi era sometimes not too good for them.
So indeed the rotation of players by taking risks like this is one way for Flick to test the readiness and mentality of the players and the results I think on several occasions of course we see that it is a very good thing.