Madrid on Thursday confirmed that tests had found a problem in Ronaldo's left hamstring, although the club declined to give an estimate of how much time he might miss.
"The tests conducted on Cristiano Ronaldo have detected a muscular injury to his left hamstring," a statement said. "This is pending developments."
Reports in Spain have suggested Ronaldo could miss around 15 days with the injury, a time span that would force him to miss the Copa final against Barcelona on April 16, as well as the first leg of the Champions League semi-final, which would be on April 22 or 23. The Champions League draw takes place on Friday.
Ronaldo sat out the second leg of Madrid's quarter-final against Borussia Dortmund, and after the match Carlo Ancelotti had claimed he was suffering only a minor problem and should be back in time for this weekend's meeting with Almeria.
"Cristiano was fine, we didn't want to risk him, because of the intensity of the tie, and we didn't have to," Ancelotti said after the game.
"Hopefully he's better soon and he is ready for the semi-final. It's not a big problem because if it were, he wouldn't have been on the bench. He has time to recover and could play on Saturday."
Ronaldo had trained on Monday before the Dortmund match but withdrew with the injury after only a few minutes.