Juventus captain Giorgio Chiellini will need surgery after damaging his anterior cruciate ligament in training.
The Italian champions have not given any timeframe on a possible return for the 35-year-old Italy centre-back, who has played 507 times for the club.
Cheillini had been named in the Italy squad for Euro 2020 qualifiers against Armenia and Finland hours earlier.
A Juve statement confirmed he sprained his right knee on Friday and would have surgery in the coming days.
Juventus signed centre-backs Matthijs de Ligt from Ajax and Merih Demiral from Sassuolo this summer after the retirement of Andrea Barzagli.
Maurizio Sarri’s side host Napoli on Saturday at 19:45 BST as they look to make it two wins from two in Serie A.