A series of medical tests have revealed the problem, and the Real Madrid forward is unlikely to feature against Galatasaray in the Champions League.
The prolific attacker was forced to leave the pitch after 54 minutes in Madrid's 5-0 Liga win over Almeria over the weekend and underwent tests Monday morning to determine the extent of the knock.
"Following a series of tests conducted by Real Madrid's medical services at the Sanitas La Moraleja hospital, Cristiano Ronaldo has been diagnosed with a lesion to the posterior compartment of his left thigh," an official Madrid statement reads.
It remains unclear how long the 28-year-old will be unavailable because of his thigh injury, but it seems unlikely that he will feature in this week's Champions League match against Galatasaray at the Santiago Bernabeu.
Madrid currently sits on top of Group B with 10 points from four games and needs a draw Wednesday to qualify for the knockout stage.