The 27-year-old sustained the problem in a collision with Levante defender David Navarro at the weekend.
The complaint has forced Ronaldo to miss Portugal's friendly encounter with Gabon this evening, but he is confident of returning to action in the near future.
"Thank you for the messages that you sent in the past days. I'm feeling better and hope to be back 100% ASAP," he wrote on Facebook.
Navarro went on record earlier this week to stress that he did not mean to hurt Ronaldo.