The 35-year-old shows no signs of slowing down and Santos believes Cristiano will quit once his physical level drops.
Speaking to TVI24, Portugal head coach Fernando Santos said: "Cristiano Ronaldo is ready to play until he's 40, but he doesn't know if it will happen.
"At some point, he may feel that he no longer has the same conditions.
"He's not a player who will lower his level when he feels like he can't be Ronaldo, he won't."
35-year-old Cristiano, who has already scored 3 goals this season, remains a key player for both Juventus and the Portugal national team.
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