The European nation are preparing for their final group stage game on Friday, with qualification for the Round of 16 already secure.
While South Korea are still competing with Uruguay and Ghana to see who will qualify, they may not have to play against Ronaldo.
Boss Fernando Santos said to reporters: “Let's see. In principle, Ronaldo will train soon.
“If you're in a position. I don't even know if it's 50/50 [the probability of being fit for the game]. If you train normally.
“Cristiano Ronaldo was replaced in both games. There's a plan in case Cristiano can't play.
“There is no player guaranteed to play at all time. It would hardly run if we didn't have a plan."