Juventus and Italy captain Giorgio Chiellini has been ruled out for up to six months with an anterior cruciate ligament (ACL) injury.
Chiellini has undergone surgery in Austria and may not be back until March.
"This afternoon (Tuesday), Giorgio Chiellini underwent surgery to reconstruct the anterior cruciate ligament of the right knee," the club confirmed on their official website.
"The intervention, performed at the Hochrum clinic in Innsbruck by Professor Christian Fink in the presence of the Juventus Club Doctor, Dr. Tzouroudis, was perfectly successful.
"The expected recovery time is around six months."
Juve have won their opening two matches of the Serie A season as they look to claim their ninth consecutive Italian title but they must now do so without one of their key performers.
They must also navigate a tough Champions League Group D which also contains Atletico Madrid, Bayer Leverkusen and Lokomotiv Moscow.
Chiellini would be on target to return to action for the quarter-finals of the competition, if Juventus make it that far.