Nearly four years after leaving for Real Madrid in a world record £80million deal, Ronaldo will be back at Old Trafford on March 5 to try and help his current team knock his old one out of the Champions League.
It is bound to rekindle memories of the brilliance Ronaldo brought to the Premier League in his six-year stint with the Red Devils, as well as trigger thoughts about whether it ever could be revived.
Ferguson has spoken optimistically about the prospect in the past - and he still won't rule it out completely.
However, in sober analysis, the Scot is not convinced it will ever happen.
"I don't see it," said Ferguson.
"I would love to think it. Maybe when he is about 39 and you are coming past my grave, but it is fanciful thinking really.
"First of all how much would it take to get him from Real Madrid?
"Secondly I don't think there is any chance they would want to sell him anyway.
"You never know. I don't know when his contract finishes and he may want to go to another club at some point in his career. Of course I would hope he'd want to come here. But that is a long way off."
Ronaldo's return is fixed though. And Ferguson knows the player he spent £12.24million on to sign from Sporting Lisbon as a 17-year-old in 2003 represents a significant threat to his side's hopes of progression in Europe's most prestigious competition.
So much so that Ferguson's first thought when he heard the draw was to check the small print of that world record transfer.
"Our contract with Real is that he can't play against us," laughed the Scot.
"I think it would be a disgrace if he played against us. It would be no loyalty."
Although Ferguson rates Ronaldo alongside Lionel Messi as one of the best two players in the world, he is reluctant to grade the 27-year-old alongside his own star men, save to confirm he is amongst the elite.
"I have been very fortunate to have had (Paul) Scholes and (Ryan) Giggs and Eric Cantona, who were all fantastic players.
"He is certainly in that bracket."
Yet Ronaldo is not the only side issue to a mouth-watering last 16 tie.
For Jose Mourinho, the man many regard as Ferguson's likely managerial successor, will be in the opposition dugout, just as he was in 2004 when his Porto side dumped United out of the Champions League on their way to winning the tournament.
In addition, United won just one of 10 matches they played against Chelsea when Mourinho was at the helm.
"I don't have a great record against Jose so I will have to put that right," said Ferguson.
"We have not had a really good European game for quite a while and this is a big opportunity to step forward in European football again.
"But the main focus will certainly centre around Cristiano.
"He has courage, like all the great players.
"Not in the sense of running into a brick wall but in the way they take the ball no matter the challenges. That is the best form of courage."