I don't understand why coaches use him. Ita just one of those cases that you can't seem to justify how a brilliant manager makes this decision time after time.
Even if you don't use players like Araujo and Gavi. This is not because they can't perform well, it's because they have different attributes than what you ask of them.
Gavi is never going to be a player that controls the midfield with passes. Both him and Araujo should be put on when the team is switching from a high press, ultra attacking plan to a more conservative sit back behind the ball and counter approach. Should be fairly clear by now to Flick.
Otherwise, it's like trying to fit a square peg in a round hole.
Sure, he can choose to only play plan A no matter what, but then don't go moaning about squad not giving you options. It does give you options, but if you only are interested in playing the one way, all the players that can bring you something different will not be of much use.
I would say Araujo, Gavi, and partly Fermin are the three players that are declining under Flick compared to Xavi. All of them would bring valuable things at specific moments in matches that would benefit us long term, but the manager is not using them out of his own, in my opinion stupid, choice.