The 27-year-old has been linked with a possible exit at Real Madrid in recent months, with Manchester City, Paris Saint-Germain and United reportedly interested in making a swoop for the Portugal star.
Ronaldo scored 118 goals for United in a six-year spell at the 19-time English champions before joining Madrid in a world record £80m deal.
Ferguson, who paid £12m to sign Ronaldo in 2003, believes the forward could quit the La Liga champions, but the veteran manager admitted most clubs would struggle to afford the former Sporting Lisbon winger’s transfer fee.
“How do you know I haven’t [inserted a buy-back clause]? He always wanted to play for Real, it was a boyhood dream, but I’m not saying he’ll stay there forever,” said Ferguson. “He might leave, you never know.
“I don’t think it’ll happen but I would like to think it would [be possible to re-sign Ronaldo].
“We’d be talking incredible amounts of money now. What do you think his value would be? He’s never injured. When does he ever miss a game? Who could afford him?
“One of the Russian clubs could buy him but do you think he’d go to Russia? He complained about the weather here so imagine him in the middle of winter in Russia!”
Ferguson has worked with a number of world-class players during his 26-year reign at United, including Eric Cantona, Ryan Giggs and Robin van Persie.
The Red Devils boss insisted Ronaldo was up there with the club’s greatest-ever players, including the likes of Sir Bobby Charlton and George Best.
“He’s an unbelievable player,” he said.
“One of the best, no doubt about that. We have had a great pantheon of players but no question Ronaldo compares with any of them. In his last season, he made five chances every match – he’s a brilliant player.”