It's officially the worst season for a Juve starter since I started following Juve.
From top of my head... Legrottaglie in 2003/04, Boumsong in serie B, Molinaro had one pretty bad season, Amauri in 2009/10, Poulsen had one terrible season, Felipe Melo was awful in 2010/11, and of course Marco Motta that same season as Melo.
Until 2019/20 Motta was with the worst season ever. From this year it's Bernardeschi. He should stop playing football after this.