The sale of David Beckham to Real Madrid has long been on a list of subjects declared off-limits by Sir Alex Ferguson, but, finally, four years after the event, the Manchester United manager has firmed up the long-held suspicion that it had rather less to do with contractual issues than with the pervasive influence of the former England captain’s pop star wife.

As recently as a fortnight ago, after Roy Keane’s tirade about football’s WAG culture, Ferguson straight-batted questions about the problems that arose from Beckham’s marriage to Victoria Adams. The mutual antipathy between Beckham’s manager at United and his wife was well known, given that the Spice Girl once joked in an interview that she would be happy to let Ferguson drown, but the Scot was widely thought to be saving his opinions for his memoirs – at least until he opened up at a remarkably candid question-and-answer session in his native Glasgow on Tuesday evening.

“He [Beckham] was never a problem until he got married,” the 65-year-old said at “An Audience with Sir Alex Ferguson” at the Citizens Theatre. “He used to go into work with the academy coaches at nighttime. He was a fantastic young lad. Getting married into that entertainment scene was a difficult thing. From that moment his life was never going to be the same. He is such a big celebrity, football is only a small part. The big part is his persona.”

Despite that persona, Ferguson believes that, when it comes to winning hearts and minds among the American sporting public after his recent move to the Los Angeles Galaxy, Beckham faces what his new celebrity friend Tom Cruise may recognise as Mission: Impossible. Major League Soccer signed Beckham on a five-year contract worth up to £128 million in an attempt to take the sport’s popularity in the United States to new levels, but Ferguson believes that the failures of the North American Soccer League in the 1970s indicate the size of the task faced by his former protégé.

“In the late 1970s I went to America with Aberdeen and there were guys like [Teófilo] Cubillas [the former Peru forward] there,” Ferguson said. “Before that there was Pelé, [Johan] Cruyff, [Franz] Beckenbauer. It is difficult with David going there. I don’t know what kind of impact he can make. David Beckham himself can’t change the whole country.

“Also, the size of the country makes it difficult. In European football, and especially in British football, you can travel easily. If you’re in Boston and need to go to Los Angeles, it’s a six-hour flight. Supporters don’t travel, so you are missing that rivalry between fans. So you have a problem. To make it substantial you would have to go regional, but there’s not enough teams to have four strong leagues.”

Ferguson also admitted that another controversial transfer, that of Jaap Stam to Lazio in 2001, was “a mistake in playing terms” and, while refusing to put a date on his retirement plans, suggested that Carlos Queiroz, his assistant at Old Trafford, would be a leading contender to succeed him when the time comes to step down.

“Thankfully, I don’t have to make the decision,” Ferguson said. “I have a great assistant in Carlos Queiroz and I think he will be a strong candidate. We have new owners now [the Glazer family] and I don’t know how they look at the situation over in America.

“They have been great supporters of everything we have done, but they may have their own ideas. The thing is I have not made my mind up when I’m retiring. It’s a difficult one. It’s not going to be an easy decision. The important thing is to leave a good team. The present team is very, very good, but they will get better.” One young player who may be part of the club’s plans postFerguson, Danny Simpson, signed a new three-year contract yesterday. The 20-year-old defender impressed on loan to Sunderland last season and is regarded by Ferguson as a possible long-term replacement for Gary Neville.