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.

Medics from his club and country last week agreed an operation was required after Ronaldo revealed the problem following Portugal's Euro 2008 exit.

"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.

After Portugal were knocked out of the European Championship by Germany in the quarter-finals, Ronaldo revealed that he had been playing with a problematic ankle for weeks and felt it required surgery.

Most ankle surgery requires six weeks rehabilitation, though 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 is at the centre of huge transfer controversy and is not set 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.

Milan could be outbid for both Adebayor and Drogba

AC Milan are still tracking Chelsea's Didier Drogba and Arsenal's Emmanuel Adebayor as they hunt for a striker, says technical director Leonardo.

Milan need a new forward after selling Alberto Gilardino to Fiorentina, while Ronaldo has a serious knee injury.

"It's no secret we're interested in signing a forward and are studying candidates," Leonardo told BBC Sport.

"Drogba is a fine player and Adebayor is a player I really like. We are waiting for the right time to move."

Milan vice-president Adriano Galliani said in June that Adebayor was his club's "sole transfer target" of the summer, although he later said the 2007 European champions could not afford the Togo striker.

On Friday, Barcelona vice-president Ferran Soriano suggested the Catalans were close to signing the 24-year-old.

Ivory Coast forward Drogba, who was set to attend Chelsea's first day of pre-season training on Monday, has been linked with Milan's city neighbours Inter as well as Spanish giants Real Madrid and Barcelona.