The 26-year-old is away with the Portugal squad who are preparing to take on Denmark in a winner-takes-all Euro 2012 qualifier.
Ronaldo is expected to start the encounter in Copenhagen, despite receiving medical advice to the contrary.
"When you play with discomfort, injury is more likely to occur. He will play and knows this, but is not injured," a member of the medical staff told Marca. "He has trouble but he will not rest. He should get some rest after the game.
"He wants to play against Denmark because he feels it is his duty, and there is no medical reason that says otherwise, but it is not good for an athlete to play too many games with discomfort.
"He should stop, but a player like him is hard to convince."
The attacker has made 84 appearances for the national team.