An ACL tear at this early age can change your career trajectory.
It doesnt mean its for the worse but it will change what kind of player he is to an extent. It may end up that he becomes less aggressive and headless chicken and focuses more on football. And his injury heals well and they manage to stabilise his reformed ACL.
Or it could be a very bad tear and get constantly reinjured. And he loses athletic ability and adds nothing.
We really don't know yet but it could be very bad or not matter much.
That's obviously a possibility, but as long as the surgery goes well and he doesn't have other injuries along with the torn ACL, and the recovery is smooth and they take their time he could return to be as he was just before the injury.
Just look at Wirtz as a recent example.
As for Gavi's development and maturity as a player then that need to be guided by his manager as well, and I suspect Xavi is fine with him being a headless chicken running around instead of being the technically good player he can be.