Real Madrid have confirmed captain Sergio Ramos sustained a Grade II sprain to the medial collateral ligament in his left knee on Spain duty on Sunday.
The centre-half landed awkwardly after contesting a header during the World Cup qualifier against Albania and had to be substituted in the 80th minute.
Team doctor Oscar Celada confirmed straight after the game in Shkoder - which Spain won 2-0 - that Ramos had damaged the ligament but hoped it would only be a short-term injury.
The former Sevilla defender underwent tests on Monday with Madrid outlining the results in a statement.
It read: "After tests on Sergio Ramos at the University Hospital Sanitas La Moraleja, he has been diagnosed with a Grade II sprain of the medial collateral ligament in his left knee. His recovery will continue to be assessed."
Reports in Spain suggested the 30-year-old is facing up to a month on the sidelines.
He could miss LaLiga games with Real Betis, Athletic Bilbao, Alaves and Leganes as well as the Champions League doubleheader against Legia Warsaw.