The 35-year-old Derby County manager - who played with Ronaldo for the Red Devils between 2004 and 2009 - thinks his former teammate can play for at least another four years because of how he's adapted his style on the pitch.
He said: "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."
Rooney - United's all-time record goalscorer - paid tribute to the 36-year-old forwards conditioning and ability to adapt to extend his own playing career.
As quoted by Sky Sports, he added: "The last time he played here, he was more of a runner, a great dribbler, with pace and power.
"Whereas now, he's a goalscorer. I think his game has adapted dramatically."
Ronaldo - who scored 188 goals for United when he was first there from 2003 to 2006 - recently made the move back on a two-year deal after a spell at Juventus.
He could make his second Old Trafford debut against Newcastle United when the two sides meet in the Premier League on Saturday (11.09.21).