Ronaldo has not yet returned to Man Utd since the end of the season
Manchester United winger Cristiano Ronaldo has undergone "successful" surgery on his injured right ankle.
The Portuguese star had the operation at the renowned Academic Medical Centre in Amsterdam, and is expected to leave hospital on Tuesday.
"Initial indications are that the procedure was successful," United said in a statement on Monday.
The 23-year-old's return to fitness is to be reviewed in a month, but he could miss the start of the new season.
Most ankle surgery requires six weeks of rehabilitation, which would take Ronaldo right up to the start of the new Premier League season on 16 August.
He was not due to travel to South Africa next week anyway for a three-match pre-season tour, and could miss their entire programme including the Community Shield against Portsmouth on 10 August.
United are reluctant to estimate when the player will get back to full fitness.
"Cristiano will begin his convalescence under the direction of club medical staff and an estimation of his return to full fitness will be possible following review by the specialist in one month," said the statement.
Ronaldo revealed after Portugal's exit from Euro 2008 exit that he had been playing with the problem for weeks.
Medics from United and the Portuguese national team agreed last week that surgery was required to resolve the problem.
Ronaldo is not due to return to Old Trafford for another fortnight while manager Sir Alex Ferguson is yet to speak to him about the statements from Spanish giants Real Madrid about the possibility of him joining them.
Ronaldo was quoted after Euro 2008 as saying it was his "dream" to move to Real, but last week United reiterated their stance that he is "not for sale" amid reports that a move was imminent.
On Sunday, Real Madrid president Ramon Calderon indicated they would only open talks with the player if United chose to sell him.
But on Monday he said that Ronaldo would be the one to decide where his future lies, and that United should be "proud" to negotiate a potential world-record transfer.
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