Reports say the 27-year-old has told family and friends he misses the Red Devils and, in particular, United boss Sir Alex Ferguson, who Ronaldo once said was his "father in sport, one of the most important factors and most influential in my career."
Whilst seemingly far-fetched on the surface, Cristiano Ronaldo back to Manchester United is a fairly logical transfer.
It's the same as the growing rumours of Jose Mourinho returning to Chelsea, with both player and coach at Real Madrid finding themselves in the same situation.
Mourinho has faced heavy criticism from the media and board members at his own club, especially after matches against FC Barcelona.
He's also been booed by his own fans, most recently in the 4-1 win over Athletic Bilbao.
And yet, he's lead his team to an unprecedented seven point gap over Barca in La Liga so far this campaign.
It's no wonder he's starting to become disillusioned with life at Real Madrid and why a return to the Premier League, especially with Chelsea, is growing ever more appealing.
After all, he's regarded as a legend at Chelsea, was simply lauded in his glory days at the club and is universally loved in England.
The same applies to Cristiano Ronaldo.
Should Cristiano Ronaldo Move Back to Manchester United?
The Portugal star has receives constant derision every time he fails to perform against Barcelona, or any other team for that matter, and is often dismissed as a flat-track bully.
Tempers were flared further in a 5-1 win over Granada last month, when sick of receiving jeers from his own team's fans, Ronaldo did not celebrate after scoring the fifth goal.
Instead he gesticulated his disdain to the crowd and put his head down as he walked back to the centre circle.
That move went down as well as a Catalan flag with the Madrid fans and pro-Madrid media, as well as with club legend and honorary president Alfredo di Stefano, who said:
The whistles against Cristiano Ronaldo? The people in the crowd are the ones who pay, and they are always right.
If the spectators demand from the players that they do their utmost and score goals, the players shouldn't question this.
And this is all despite the player having adapted extremely well following his world record £80 million transfer.
He started scoring almost instantly and is the second-most prolific marksman in the world behind only Lionel Messi, having scored 31 goals this season from 32 games, as well as a staggering 117 in just 121 matches for the club overall.
It seems as if, despite all he's done for Real Madrid, the fans and the club itself don't truly care about their star player.
He scored at a much lower rate at United, having scored more than 20 Premier League goals in only one of his six seasons at Old Trafford, yet was still heralded as a legend by the Red Devils and is still loved by virtually all at the club.
So like Mourinho, it's clear that a move back to England is starting to seem ever more appealing for Ronaldo.
And no doubt the player would also take encouragement from the return of Paul Scholes, with the joy over his United comeback there for all to see.
It's even clear to see how the return of Steven Pienaar and Landon Donovan to Everton has excited not just those on the blue half of Merseyside, but the Premier League as a whole.
The prospect of a Manchester United return, compared to his current dilemma at Los Merengues, surely must seem enticing.
After all, this is a player who does make trips back to England quite frequently and did so again just before Christmas, where he attended the ATP World Tour final between Roger Federer and Jo-Wilfriend Tsonga in London, along with former teammate and international colleague Nani, as well as Chris Smalling from United and former Premier League rivals Gareth Bale and Andrei Arshavin.
Earlier that day in England's capital, Ronaldo raised questions over a United return when he was seen at Les Ambassadeurs, an exclusive club near Hyde Park.
He was spotted with agent Jorge Mendes, and Manchester United CEO David Gill, a man who he has developed a strong relationship with over the years.
In 2012, a year that's turning out to be the one of returning legends, could Ronaldo provide the world with its most sensational comeback yet?
The only sticking point would be a fee, but should United give back the £80 million they received for their talisman, a deal could be on the cards.
A phenomenal player entering his prime, Cristiano Ronaldo isn't given the love or respect he deserves at Real Madrid.
At Manchester United however, three seasons after his departure and that love and respect hasn't faded, despite the fact he's doing better in Spain than he ever did in England.
So surely Cristiano, to get the appreciation you truly deserve, why not return home to Manchester?
It's got to be better than being booed for scoring goals.