Can we stop the bullshit complaining about zonal marking? Zonal marking is only used on set pieces anyway.
The problem is lack of width which leads to fullbacks being outnumbered on the wings. From there it's easy to notice that opponents have it way too easy when it comes to crossing into Juve's box. You can't give so much space to wingers/fullbacks when they are about to cross.
Funnily enough it worked well last season when Matuidi would support Alex Sandro on the left. Yesterday Matuidi was so useless, he was nowhere to be seen on either end, leaving AS alone to cover the whole flank.