He can play even as an interior in Rakitic's mold (except better in link-up and with not as much scoring ability, so less box to box and more DM-CM type) but he needs a proper manager to give him smarter instructions. If even that doesn't make his performances go up a notch or two, selling him while we can for big bucks is not a bad option.
It's hard fot me to describe.
When I talk about Frenkie being bad in possession:
1. he moves too much, even though that sounds weird.
But he moves in a chaotic way and he is not there where you expect it.
Rakitic moved too little for Barca's standards, but at least you knew that he will be on his position.
So, if you passed the ball towards his default position, he was there.
Even though, after that pass to him, he lacked a movement off the ball once he released the ball.
On the other hand, Frenkie runs all the time.
Which is good on one hand, but is bad in terms that he is always on a different position.
The easiest comparison would be, imagine if Xavi-Iniesta-Busi needed to play blindfold.
Xavi would know even without looking where Busi is.
Busi would always know where is Xavi.
Now, imagine the same situation with Frenkie.
When Busi has the ball (blindfold), he has no clue where is Frenkie.
Sometimes he stays on his RCM position, sometimes he wanders to a wing position, sometimes he wanders to a CB-pivot position, sometimes he wanders into the box.
So, for example, in 433 with Xavi-Iniesta-Busi, you had 3 clear midfielders who interchanged positions, but who were always "on the right position" positionally wise and open for receiving passes.
On the other hand, with Frenkie in midfield, you get 423 formation and Frenkie is everywhere, on a CB, pivot, RCM, LCM, RW, false 9.
On one hand, he is everywhere.
On the other hand, he is nowhere when you need him to keep possession and dominate the other team.
Again, I don't think that it is a coincidence that we declined a lot in midfield since he became a starter.
We gained a lot of headless chicken running for 90 minutes, but gained very little smart movement, smart possession, smart defending or anything creative from him.
A weird, weird player.
When you look at him, he looks like a world class, but when you dig deeper, he does almost nothing except running around aimlessly for 90 minutes.