The Portuguese limped off at the start of the second half of Saturday's La Liga victory over Almeria but fears of a serious injury were immediately played down by coach Carlo Ancelotti afterwards.
Ronaldo left a medical examination on Monday morning telling reporters "I am fine", but the club released a statement saying: "there is an injury to the back of the thigh which will be monitored."
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Ronaldo has been in fine form recently with 14 goals in seven games for his club and country and the injury comes after a demanding start to the season where he has played every match so far.
He could now well miss the Galatasaray match where anything less than a heavy defeat will see them qualify in first place from Group B.
Image: Cristiano Ronaldo of Real Madrid competes for the ball with Jesus Saez de la Torre of Almeria during the La Liga match
Photograph: Gonzalo Arroyo Moreno/Getty Images
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