Numerous media outlets recently claimed that the four-time Ballon d'Or winner is keen to quit the Bernabeu due to the tax case looming over him.
Spanish prosecutors have alleged that Ronaldo has avoided paying €14.8m (£12.9m) in taxes relating to image rights.
The prolific attacker has denied the accusations, but it has led him to reconsider his future, with a return to United thought to be on his mind.
However, according to The Express, United are wary about the situation and fear that the 32-year-old is using them in order to get an improved deal from Madrid.
Manager Jose Mourinho had a difficult relationship with Ronaldo during the Portuguese coach's three-year spell at the Bernabeu, and it is believed that the Champions League winner was not initially a target drawn up on the transfer wishlist.
The report claims that despite the player's quality, there are concerns that he would not fit into the team that Mourinho is trying to mould.
United are also worried that a long-running transfer saga involving Ronaldo could result in the club missing out on other potential signings.