A new manager won't help in this case.
Barca needs to grasp that times changed and the contemporary high-class game is based on execution-speed and on speed as such.
Having the boring and absolutely counterproductive possession, even when the opponent is opened up, having twice as much slow passes as the winner opponent often has, leads nowhere anymore.
Giving safety passes and the like, getting forward only when the unnecessary passes are almost unbearable anymore, represent the past of this game.
The two central defenders are slow and clumsy, Busquests looks like a grandpa (with still considerable technique - but who cares), Griezmann is slow and exceptionally one-footed, not a good and reliable finisher at all, Pedri is really talented but his time comes in 2-3 years, nobody wants, dares or can shoot from a distance (Rakitic was at least brave and occasionally good in it), Dembele has no space because he always faces a solid stationary defense due to this outdated style, no player can serve Messi on his level needed, so essential problems are piling up.
A new coach will not be able to teach a modern, fast, effective and efficient game to this guys (who play the very same way against a 3rd league copa opponent), because for who knows how many years they have been told that mere possession is the "Barca style"...