Koulibaly instantly improves our defence but wonder how Xavi rotates him, Piqué, Araujo, Eric & Christensen.
If Piqué retains his top form, he's fundamental, Araujo also non-negotiable & Koulibaly the obvious star reinforcement.
Barça would go from thin to stacked in a window.

But you have to wonder - do we need five CBs?
If we're getting Koulibaly, is Christensen even necessary? Of course, we'll likely lose Piqué after a year so we'd be down to 4.
But next season, someone will likely not see much game time.
One of Eric & Christensen will suffer.

This obviously affects Eric the most.
I'm still certain he can reach the level we need him to but only with enough game time.
He needs minutes to progress, as any young talent does. It's fundamental.
But how much will he get next to Araujo, Piqué & Koulibaly?
Tough spot.

On paper I'd agree Koulibaly's transition to our best defender should be seamless but you never know.
Form should always dictate who starts games. Also, I forgot about the 3atb - definitely a possibility.
Don't disrespect Piqué, though. When fit & in form, he's immense.