The 41-year-old superstar was injured in Saturday's 3-1 win against al-Fayha.
"He has started a rehabilitation program and will be evaluated day by day," the club said in a press release.
Since 2014, Cristiano Ronaldo has had a chronic hamstring injury in his left leg, Spanish outlet Marca writes. However, it is not clear whether it is the same leg now.
Ronaldo's Portugal is ready for the World Cup, which begins on June 11.
