Three months on, and with one more World Player of the Year under his belt than eternal rival Lionel Messi had managed at that time, the Portuguese had swapped red for white as he made a goal-scoring bow for Real Madrid against Deportivo La Coruna.
That debut was a sign of things to come, with a prolific spell in the Spanish capital having sent records tumbling and raised the bar of when it comes to individual consistency.
Ronaldo now has 413 goals for Real in just 403 appearances, two La Liga titles, three Champions League crowns, two UEFA Super Cups, two Club World Cups, a European Championship winners’ medal with Portugal and three more Ballon d’Ors on his mantelpiece.
Having emerged as a force of nature at Old Trafford, the 32-year-old has become a global phenomenon at Santiago Bernabeu.
United have fared okay without him – two Premier League titles, two League Cups, the FA Cup and Europa League have been secured since he left – but it has taken far more than the £80 million generated by his sale and several changes of manager to get back to somewhere close to that level.
Jose Mourinho, who spent three years with Ronaldo in Madrid, has the Red Devils looking more like the Sir Alex Ferguson-inspired teams of old, with another dose of Portuguese magic being sprinkled over the Theatre of Dreams.
There is, however, only so much he can do from the touchline, with it up to those on the field to recapture the glory days of old.
One man helping to lead that charge is David de Gea, with the Spanish goalkeeper having been a model of consistency throughout a period of much change and uncertainty for United.
Three Player of the Year awards and four appearances in the PFA Team of the Year help to highlight just how important the 26-year-old has been and continues to be for the Red Devils – with those achievements matching Ronaldo (although he did scoop two PFA Player of the Year awards as well).
De Gea’s brilliance has not only been recognised by those inside Old Trafford, it has sparked debate similar to that which continues to swirl ferociously around Ronaldo, Messi and their seemingly endless battle to outdo one another.
Poll a group of supporters pretty much anywhere in the world and you will get divided opinion regarding who comes out on top.
Ask those same people to name the best goalkeeper in the world and you are likely to get a similar split – with De Gea now widely acknowledged to be the closest challenger to, if not on par with, Bayern Munich’s World Cup-winning Manuel Neuer.
Many now even consider him to be the pick of the bunch in his chosen position.
Gary Neville told Sky Sports after seeing De Gea produce another point-saving stop to earn United a 0-0 draw at Liverpool: "Is there anybody better now in the world than David de Gea? I'm not sure. [Thibaut] Courtois I love to pieces, but this guy is just doing things that are absolutely stunning. He is a brilliant, brilliant goalkeeper. The save there and the one at Stoke a few weeks ago were absolutely outstanding. You need that. I don't think you can win things if you haven't got a good goalkeeper."
Ashley Young has echoed those sentiments, telling MUTV: "I don't think there is [anyone better]. I said a few seasons ago I thought he could be the world number one and I think he's shown that he is now."
This season alone, nobody in the Premier League has kept more clean sheets than De Gea (seven) or kept out more ‘big chances’ for the opposition (five), while none of those at fellow title-chasers – Courtois, Hugo Lloris, Ederson et al - can better his save percentage of 90.91, with Burnley’s Nick Pope (91.67) the only man ahead of him.
Those figures are mirrored across his entire spell in England, with a division-high 82 shutouts secured, a save percentage of 73.53 and only six errors made leading to shots on his goal – a statistic that no-one to have played anywhere near his number of games (208 and counting) can challenge.
His value is clear for all to see, with comparisons drawn to United legend Peter Schmeichel.
Unfortunately, among those to have taken note are Real, with the Spanish giants forever in the market for ‘Galactico’ talent – especially when said stars are Madrid natives and ones apparently open to a homecoming at some stage.
The latest rumours – following several failed attempts to get a deal over the line in the past – suggest that the Blancos will be back in 2018, with Florentino Perez reluctant to let United resistance and dodgy fax machines leave him empty-handed.
Talk of a possible exchange deal involving Gareth Bale makes little sense, given that the Red Devils would be trading a virtual ever-present in a key position for an injury-ravaged performer in an area of the field in which they are already well-stocked, with United hardly in need of countering the cost of any move for the Welshman with a makeweight.
They do not need the money, they need the talent.
De Gea is among the best in the business at the moment and like Ronaldo before him, is approaching what should be the peak years of his career.
To allow another world-class performer to slip through their grasp and head for the Spanish capital would be difficult to stomach, as a player that so much time and faith has been invested in goes on to flourish even further in pastures new.
That is sometimes the way of the world, with player power continuing to rise, but until the day comes that De Gea openly admits that he wants to head home, United must do everything in their power to ensure that there is no repeat of the Ronaldo episode with Real Madrid.