Ramos, 30, 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 the centre-half 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.”
He could miss LaLiga games with Real Betis, Athletic Bilbao, Alaves and Leganes as well as the Champions League doubleheader against Legia Warsaw.