Ronaldo was replaced early in the second half of Saturday's Primera Division win over Almeria with what coach Carlo Ancelotti described afterwards as only "a little problem" and the £80million forward underwent scans on Monday morning.
Those tests revealed Ronaldo has an injury and Madrid must now wait to see how the 28-year-old recovers.
A Madrid medical statement read: "After the tests taken today by Cristiano Ronaldo by the Real Madrid medical services in the Sanitas La Moraleja hospital, an injury has been detected in the back of the left thigh. Pending developments."
According to reports in Spain, Ronaldo told the waiting press "I'm fine" as he left the hospital following his scan but it remains to be seen when he will be available for Madrid again.
Ancelotti's men are back in action on Wednesday night in the Champions League when they take on Galatasaray at the Bernabeu, where the nine-time champions will look to seal their spot in the last 16.
Madrid then host Real Valladolid in the Primera Division on Saturday before taking on minnows Olimpic Xativa the following weekend in the first leg of the Copa del Rey last 32.
Ronaldo has been in phenomenal form this season and took his tally to 35 goals in his last 25 appearances in all competitions with the opening goal in the 5-0 win over Almeria.
He has scored 67 goals for club and country in 2013.