United great Ronaldo left Old Trafford for the Bernabeu in a then record-breaking £80million deal in 2009.
But the Red Devils have been regularly linked with a move to bring the Portuguese superstar back to Manchester.
And The Sun claim United are monitoring Ronaldo's situation and will make a bid if they are given a green light from the player.
Ronaldo is apparently unsettled in Madrid following the appointment of former Liverpool boss Rafa Benitez.
Manchester United reportedly want to bring Cristiano Ronaldo back to the Premier League
The 30-year-old was caught swearing at the Spaniard during a training session and is also quoted as saying: "You always only pick on the Portuguese."
Only last week Benitez risked the wrath of Ronaldo by refusing to single him out as the best player in the world.
"It is difficult to say who's best," he said. "I think Cristiano, Gareth Bale, Karim Benzema or James Rodriguez are at that level."
However last night the 55-year-old backtracked somewhat, saying: "Fortunately, after working with him, you understand how competitive he is, how much he likes to win, the quality he has. For that, he's the best in the world.
"I would like to see everyone have the same desire to win."