Juventus and Italy midfielder Claudio Marchisio will likely miss the rest of the season after suffering a ruptured cruciate ligament in training for his country on Friday.
Marchisio sustained the blow preparing for Italy's Euro 2016 qualifier with Bulgaria on Saturday.
The initial diagnosis said that Marchisio had sprained his right knee.
But a statement from Italy doctor Enrico Castellacci released by the Italian Football Federation (FIGC) read: "During the warm-up the player was believed to be suffering from just a sprain to his right knee.
"An MRI scan was performed immediately at the Fanfani Institute in Florence, which showed a rupture of the anterior cruciate ligament.
"After consulting with the medical staff of Juventus, it was decided that the player will return to his club for treatment."
The news comes as a major blow to Serie A leaders Juve ahead of their UEFA Champions League quarter-final tie with Monaco next month.
Juve are already missing midfielder Paul Pobga through injury and will now have to cope without Marchisio in their bid to reach the last four.