The Real Madrid forward, who helped his side clinch both La Liga and the Champions League crown this season, spent six seasons with the Red Devils.
He moved to Los Blancos back in 2009 – breaking a lot of Man United hearts in the process – and went on to enjoy incredible success.
However, the 32-year-old, who has become disillusioned by recent tax fraud allegations, which he refutes, may now be looking for a way out of Spain.
And United are just one of a handful of clubs monitoring the situation.
When the Portuguese press made the original claim, Red Devils supporters took to social media to voice their opinion.
Some suggested the club should drop all interest in other potential summer deals and put every effort into re-signing Ronaldo.
Others confidently claimed that United would be the only club he would consider joining if he was indeed looking for a way out of Spain.
While a few even suggested that the move could actually have a negative impact on them signing Alvaro Morata from Real Madrid, which looked to have all been tied up.
It is therefore a turbulent time for United fans, not entirely sure what is what at the moment in regards to who is coming and going at Old Trafford.
And it’s only going to get more interesting as the transfer window opens.
Now Ronaldo isn’t one to shy away from social media. The Portuguese has built an empire for himself and his face is one of the most recognisable in the world.
He knows what he is doing on Instagram, for example, where he boasts a staggering 104 million followers.
So he would have known that changing his display picture to one of him in a Portugal shirt could spark all sorts of rumours.
Sure, he probably just did it because his nation are currently playing in the Confederations Cup. And it’s not 100 per cent clear he was indeed wearing a Real shirt prior to changing his display picture, like United fans are suggesting.
But that hasn’t stopped them hoping for the best and Real supporters fearing the worst.