The 65-year-old gaffer added that the Portuguese captain is physically strong enough to continue playing until his body slows down.
Cristiano Ronaldo, 35, registered his 450th league goals as he helped salvaged a 2-2 draw with Roma despite playing most part of the game with ten men.
Santos told TVI24:
“Cristiano 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.”
Ronaldo has come out of the blocks firing this season, with five goals in his opening three games for club and country.