You can afford to have average or slow central defenders if your fullbacks are pacy. When playing possession football, you need that pace to avoid counter attacks. Especially in Champions League, where the big guns have incredible fast players.
Given how high up the pitch fullbacks tend to get now when a team has possession, they are frequently pretty irrelevant when it comes to defending counter-attacks.
The pace of centre-halves and the deep midfielder(s) is far more significant with regards to counter-attacks.
Where quick fullbacks are great is that you can leave them to play against an opponent one on one, rather than feeling the need to protect them as you might with a slower player.
That gives you the chance to maintain the shape of the team better.
Given Juve's inclination towards 4-3-1-2, a system that naturally leaves the fullbacks pretty exposed, you need players who can deal with wingers with limited support from the rest of the team.
