The 34-year-old does not quite have the pace of old but showed with his hat-trick in the 3-1 Nations League win over Switzerland on Wednesday that he remains deadly around the penalty area and has a knack of scoring goals that matter.
He can also embarrass opponents with his mesmerizing footwork as Swiss fullback Kevin Mbabu found out when he was left on the floor by a Ronaldo dribble.
Ronaldo, whose side face the Netherlands in Sunday's Nations League final, is now only 12 short of his 100th international goal and could conceivably reach 200 international caps, having amassed 157 appearances in his 16-year international career.
Having remained largely free of injury, there indeed seems no reason why he cannot keep going for several years, especially as he has already shown he can adapt his game to suit his age.
"I think he will play for another three or four years," said Santos, who took charge at Sporting in 2003 shortly before Ronaldo left the club for Manchester United.
"I'm not surprised by him any more," Santos told reporters. "I was surprised when I first saw him at Sporting as a youngster. At that moment, we saw his quality and potential.
"He has maintained this quality at the age of 34 and he has very well-defined and strong aims... he is in great physical shape, he is always determined and he manages to maintain his intensity between games.
"It is not very common for a player to have reached 34 and scored around 50 goals in each season, but he is very ambitious."
At one stage, Santos became frustrated at the number of questions about Ronaldo.
"By the way, we are going to play against the Netherlands but I will answer another one," he said.
Ronaldo scored 28 goals in all competitions in his first season for Juventus this season. By his standards, it was almost lacklustre --- the total was his lowest since 2008-09 when he was still at Manchester United -- although he still added a Serie A winners' medal to his collection.
Sunday's match comes 15 years after he also played for Portugal in a final on home soil, when they lost 2-1 at home to Greece in the decisive match of Euro 2004.
Ronaldo himself told UEFA's website that his preparation and work ethic had helped him defy his age.
"I still feel good," he said. "The most important thing is your head, to feel motivated and happy, and to follow my path as a player, because I think I still have a lot to give and I feel very good."
(Writing by Brian Homewood; editing by Tony Lawrence)
