"Cristiano Ronaldo was subjected to a medical check before today's training session. Results show an inflamed tendon on his right ankle," it said.
"The player will follow a medical treatment and have physiotherapy, and will be monitored for the next 24-48 hours," it added.
The 25-year-old striker took part in Real's humiliating 5-0 loss away to arch-rivals Barcelona on Monday. The injury means he may be forced to miss Real's next league match on Saturday against Valencia.
Ronaldo, the 2008 FIFA World Player of the Year, has started in all 13 league matches this season as well as all five UEFA Champions League matches, having scored a total of 18 goals.
Real are in second place in the league with 32 points, two fewer than Barcelona, following their loss to the Catalan side.
