Ronaldo began a new phase in his career on Monday night when he made his debut for Real Madrid in a scrappy 1-0 win over Shamrock Rovers. Real paid a world record £80million to let Ronaldo fulfil his childhood dream and turn out for the Madrid giants.
But Ferguson said: "He may come back, you never know. Sometimes it happens that way. He loved United. If you look back at the six years he had with us, he never missed training."
He added: "That's one of the things he has above all other players - he was never injured, he always played.
"And his best is yet to come. He is only 24."
Many United fans have already claimed they are happy to see Ronaldo go given his increasingly individual approach, particularly as it allows Wayne Rooney
to resume the central attacking berth he craves.
However, Ferguson is not joining those now attempting to rewrite history.
Describing Ronaldo's exit as "a massive loss", the United manager paid a glowing tribute to the player he still regards as number one on the planet, even if the title which currently belongs to the former Red Devil is likely to be inherited by Lionel Messi this winter.
"Cristiano is a massive loss," said Ferguson. "I have nothing but praise for the boy. He is easily the best player in the world. He is better than Kaka and better than Messi. He is streets ahead of them all.
"His contribution as a goal threat is unbelievable. His stats are incredible. Strikes at goal, attempts on goal, raids into the penalty box, headers. It is all there. Absolutely astounding."