Real Madrid defender Pepe suffered a double rib break in Saturday's 1-1 draw with Atletico Madrid, the LaLiga leaders have confirmed.
The centre-back opened the scoring in the derby at the Santiago Bernabeu but was replaced by Nacho in the 67th minute after a collision with team-mate Toni Kroos on the edge of the area.
Pepe was sent for hospital tests, which have confirmed the extent of the damage.
"Following the tests carried out on Pepe at the Sanitas la Moraleja University Hospital, the player has been diagnosed with fractures to his seventh and eighth left ribs," a Madrid statement confirmed.
"His recovery will continue to be assessed."
The news is a major blow to head coach Zinedine Zidane, with Madrid already missing Raphael Varane due to a thigh problem.
It means that captain Sergio Ramos is likely to be partnered by Nacho when Madrid face Bayern Munich in the first leg of the Champions League quarter-final next week.
Pepe is almost certain to miss both Bayern games and the Clasico against Barcelona on April 23, with early reports in Spain suggesting he could be out for the rest of the season.