In my view, Juve have a CB backup in Bonucci. He hasn't demonstrated that he is good enough to be starting.
I think the problem in Juve's last years is that we've always tried to build the team from the top (attack) to the bottom (defence), when I think the other way would be more logical and productive.
A team with a rock solid defence, is guaranteed to pick more points than a team who can't score. Because if you fail to score and fail to concede you always get a point. But if you fail score and concede, you're always losing matches.