Juve has poor defense now. After Sarri has come no one in the black-and-white team can understand how to play 3 or 4 defenders and what the mid-defenders should do to protect the second line.
I don't agree with this statement. There are several reasons why Juve is suffering in the defensive part but it is not due to the lack of quality players.
Chiellini has suffered a serious ligament injury and he is out until March 2020. That leaves the team with only one experienced defender left - Bonucci.
De Ligt was one of the best players in Ajax but that doesn't count for anything in Italy. If Chiellini didn't get injured De Ligt would have slowly been integrated into the team by working and understanding the setup in training. He wouldn't have been a regular starter in the first couple of months. With Giorgio's injury Juve was forced to play De Ligt who was not up to the task. But he has improved a lot in the last two games and he is in a much better form now.
Juve also has problems on the wings. They sold Cancelo and got Danilo at the end of the transfer period. Danilo also needs some time to adapt. Their other right back - De Sciglio got injured followed by an injury to Danilo as well which leaves the team without a single proper defender on the right. Cuadrado is now filling that role. And on the left they only have Alex Sandro without a substitute left back. He played every single minute of every game until his father passed away last week and he returned home to Brazil to bury his father.