"Ronaldo has been diagnosed with a muscle strain in his right thigh. His recovery will continue to be assessed,” Real said in a statement on Friday.
Spanish newspaper Marca reported that the prolific striker has been ruled out of Madrid's La Liga game at Rayo Vallecano on Saturday.
The Portuguese, who is the top scorer in La Liga with 31 goals, was seen limping off clutching his thigh at the end of Real Madrid's 3-0 win over Villarreal on Wednesday.
Image: Cristiano Ronaldo reacts after getting injured during the La Liga match against Villarreal.
Photograph: Juan Medina/Reuters