Ronaldo continues to bang in the goals for club and country at the age of 35, and he has showed no signs of slowing down despite picking up niggling injuries.
Clegg, who was part of Sir Alex Ferguson's coaching staff, reveals Ronaldo was the only player at United who could measure up to Ryan Giggs' work ethic off the field.
"Before Ronaldo, the best athlete in terms of work on his body was Ryan Giggs," Clegg told Gazzetta dello Sport.
"He was the strongest, the most attentive to details. If Giggs played at United until he was 40, Ronaldo can play till he is 41.
"Cristiano was the only one able to go beyond the high standards of Giggs."
10. Teddy Sheringham – retired at 42
9. Rivaldo - retired at 43
8. Faryd Mondragon – retired at 43
7. Marco Ballotta - retired at 43
6. Aleksandar Duric - retired at 44
5. Andrea Pierobon – retired at 46
4. John Burridge – retired at 46
3. Essam El-Hadary – retired at 47
2. Stanley Matthews - retired at 50
1. Kazuyoshi Miura – Still playing at the age of 53