Cristiano Ronaldo has been advised to have surgery on his right ankle this weekend and is expected to miss the start of the new season, a development that threatens to complicate his potential transfer from Manchester United to Real Madrid. The United medical staff have recommended the operation, but officials at Old Trafford have rejected the conspiracy theories that the move is designed to scupper the player’s hopes of a switch to Spain.

Ronaldo will see a specialist in the next 24 hours, having been advised by Steve McNally, the United doctor, that an operation is required and that he would be unlikely to be back to full fitness when the Barclays Premier League campaign starts on August 16.

That would raise doubts about how far Real might go to try to sign him before the transfer window closes on August 31, but McNally’s meeting with Ronaldo in Lisbon on Wednesday afternoon was also attended by Jorge Mendes, the player’s agent, and, more significantly, by Henrique Jones and Antonio Gaspar, the doctor and the physiotherapist, respectively, of the Portugal national team. All parties agreed that Ronaldo requires an operation, having played with and aggravated the injury during the European Championship finals last month.

Real are unlikely to be deterred in their pursuit of Ronaldo — having tracked his namesake, the Brazil forward, for months while he was injured in the summer of 2002 before signing him from Inter Milan just hours before the transfer deadline — but it is a situation that adds to the intrigue as United attempt to hold on to their most prized asset.

Sir Alex Ferguson, the manager, remains opposed to selling the 23-year-old, as do David Gill, the chief executive, and, contrary to reports in Spain yesterday, the Glazer family, who own the club.

Real are willing to pay a world-record fee in excess of £50 million to sign Ronaldo — and, according to sources in Spain, would be prepared to bid more than £80 million to get their man — but United maintain that he is not for sale. They have appeared to be in denial about Ronaldo’s intentions for much of the summer, but now that he has spoken of his “dream” of moving to Real, their stance has hardened.

There is a point of principle involved, with the club determined to hold the player to a £100,000-a-week, five-year contract that he signed only 15 months ago, but there is also a strong element of pragmatism, with Ferguson aware that selling Ronaldo would be a backward step for the team who won the Premier League and Champions League last season.

United are convinced that they will keep hold of Ronaldo, but much will hinge on how far the player is prepared to go in order to get the transfer he craves. He has told friends that he will do “whatever it takes” to engineer a switch to Spain, but Ferguson considers it significant that Ronaldo has not released the statement that he planned to make almost a fortnight ago in order to accelerate his move. The view within Old Trafford is that his transfer simply cannot and will not happen, whatever the money offered by Real.

United are more cautious on the future of Carlos Queiroz, the assistant manager, who has told Ferguson and Gill that he is interested in a proposal to succeed Luiz Felipe Scolari as Portugal coach, which was put to him on Tuesday.

Reports in Portugal suggest that Queiroz has already accepted a four-year contract with the Portuguese Football Federation, but United believe he may yet be tempted to stay if they can match the financial terms on offer.

The 55-year-old is understood to be more interested in securing assurances about his prospects of succeeding Ferguson as manager when the Scot retires in the next two or three years, but United, even though they hold Queiroz in the highest regard, would be reluctant to make any promises.