The France striker missed Saturday's 3-1 win over Malaga with the problem and looks set to be sidelined for the second leg of Real's Champions League quarter-final.
"Following tests in the Hospital Sanitas La Moraleja by the Medical Services of Real Madrid, Karim Benzema has been diagnosed with a sprained medial collateral ligament in his right knee," a club statement read.
Carlo Ancelotti is expected to offer further injury updates on Gareth Bale and Luka Modric at Tuesday's news conference.
The trio would be major absences for the Italian, whose side drew 0-0 with Atletico in the first leg last week.
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