Ronaldo, 36, still has the qualities to compete at higher level and being stunned by the former Sporting player's abilities, Rooney believes Ronaldo has the requisite to continue playing until he's 40, like Ryan Giggs.
"The last time he played here, he was more of a runner, a great dribbler, with pace and power," said Rooney, now managing Derby County in the Championship.
"Whereas now, he's a goalscorer. I think his game has adapted dramatically."
Rooney who played with Ronaldo at Man United stressed the Portuguese is "completely different player" compared the other old players who left the club 12 years ago and can still catch up with the high demands from the Premier League.
"His ability is obviously one thing, but then he's also looked after himself massively... he's still in great condition. It wouldn't surprise me if he's playing until he's 40, like Ryan Giggs did, and still scoring goals.
"His team-mates will have to work a little harder. But if it's anything like last time - towards the last two years of playing with him - the team allowed him to do that because he scores you goals, and goals win you games."
Ronaldo completed a £19.8 million move to United from Juventus earlier this week, having left Old Trafford for Real Madrid in 2009 for a then-world record fee of £80 million.
The Portuguese forward will be wearing his iconic number 7 shirt in his second spell at Old Trafford.
Ronaldo's arrival could power the Red Devils to a first Premier League title since Sir Alex Ferguson left the club.
Ronaldo is due to feature for Man United when the they host Newcastle on Saturday.
