According to US magazine Sports Illustrated, the Portugal forward will seek to join a Major League Soccer franchise in 2018, after the Russia World Cup.
Ronaldo, 30, was widely expected to return to the Premier League, where he played between 2003 and 2009, and sign a deal with United.
Chelsea and Manchester City are also reportedly interested in the global superstar, whose commercial pulling power is a great asset to whichever team he plays for.
Madrid have already enjoyed the fruits of his success on and off the field since signing him from United for £80million six years ago.
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Cristiano Ronaldo is said to be ready to leave Real Madrid by 2018
Ronaldo has been tipped to soon leave Real, with Barcelona legend Hristo Stoichkov yesterday saying: "Sooner or later, Cristiano will be sold by his president.
"The world of football is like that, but it depends on his president.
"I don't know if he is short of money. If he is, it's a good moment to sell him."
And the attacker would not be the first to see out the twilight of his career in the United States. David Beckham left Madrid for LA Galaxy back in 2007 and was closely followed by Arsenal's Thierry Henry, who went to New York Red Bulls.
Frank Lampard will join emerging franchise New York City this summer and Steven Gerrard is heading to Galaxy on a £6m, 18-month deal.
And reports claim Ronaldo would likely choose franchises in Los Angeles, New York or Miami should he move.
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