Sir Alex Ferguson is believed to be exploring the possibility of buying Cristiano Ronaldo.
The Portuguese star is reportedly unhappy at Real Madrid and has hinted that he could be on the way out of the club.
The Scot is said to be weighing up a potential bid for the 27-year-old forward.
"The manager made it clear when Ronaldo joined Real that if he ever left the Bernabeu he should return to Old Trafford," an insider told The Mirror.
"It's a transfer that would prove that United can still compete with anyone for the biggest names in football, but, at the moment, only Ronaldo knows whether he is serious about leaving Madrid or whether he's just having a sulk."
Ronaldo left the Red Devils to join Los Blancos for £80m in 2009.
