Juventus star Federico Chiesa will undergo surgery in the coming days on an anterior cruciate ligament injury.
The Italy international was hurt in the thrilling 4-3 Serie A victory over Jose Mourinho's Roma on Sunday.
The winger suffered what Juve described as "a blunt trauma sprain" to his left knee, which left him requiring crutches to attend the club's medical center on Monday.
"The diagnostic tests performed this morning at J|Medical revealed an injury of the anterior cruciate ligament," Juve's statement said.
"It will be necessary for the player to undergo surgery in the next few days."
Chiesa is likely to miss the rest of the season in what is a huge blow to Massimiliano Allegri's side.
The Bianconeri, who face Inter in the Supercoppa Italiana on Wednesday, are 11 points off the league-leading champions and three outside the top four.
They continue their Coppa Italia defense against Sampdoria next week, while their Champions League last-16 tie with Villarreal begins in February.
Chiesa's injury is also a significant setback for Italy's hopes of qualifying for the 2022 World Cup.
The European champions face North Macedonia in the play-off semi-finals in March and will then meet either Turkey or Portugal for a place in Qatar.
Chiesa has four goals and two assists in 18 appearances for Juve in all competitions this season, his dynamic displays with the ball at his feet often a notable bright spot in a difficult campaign for Allegri.
He has the most overall take-ons (68) and take-ons in the opponents' half (51) of any Juve player this term, while his average carry progress of 7.8 meters per attempt is the best of anyone in the squad to make more than one appearance.
In total, 23 of his carries – where a player moves five meters or more with the ball – have ended in either a shot or a chance created. Among Juve players, only Alvaro Morata, who has played seven more matches, can better this figure (28).