The France international was left out of Rafael Benitez's squad for this week's Audi Cup after breaking down in training ahead of the Blancos' departure for Munich.
Benzema now faces a race against time to be fit for the start of the capital club's Liga campaign, at Sporting Gijon on August 23.
"Following tests carried out on Karim Benzema by the Real Madrid Medical Services at the Sanitas La Moraleja Hospital today, he has been diagnosed with a muscular injury in his right thigh. His condition will be evaluated," read a statement released on the Blancos' official website.
Benzema has been heavily linked with a move to Arsenal before the close of the summer transfer window but Benitez stated on Monday that he expects the forward to remain at the Santiago Bernabeu.
The 27-year-old joined Madrid from Lyon in 2009 for an initial fee of €35 million and has since scored 87 times in 188 Liga appearances for the Spanish side.
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