The France captain has left the national team, having been forced to come off shortly after scoring the winner in Friday's 1-0 Nations League victory over Turkey.
The 27-year-old was treated after the goal and briefly returned to the pitch but went down again moments later and was unable to continue, prompting France coach Zinedine Zidane to replace him in the 58th minute of his first game in charge.
"After the tests carried out today on our player Kylian Mbappe by Real Madrid's Medical Services, he has been diagnosed with a hyperextension of the posterior capsule of his left knee," read the statement.
The French Football Federation initially said Mbappe was suffering from a "knee tendon injury" and had returned to Madrid for further assessment.
Real next host Villarreal in LaLiga on October 10th.
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