Sir Alex Ferguson tonight lost his biggest ally in the battle to keep Cristiano Ronaldo at Manchester United after Carlos Queiroz quit to become Portugal boss.

Ferguson's right-hand man at Old Trafford for all but one of the last six years, Queiroz has agreed a four-year contract with the Portuguese Football Federation to succeed new Chelsea boss Luiz Felipe Scolari.

Queiroz, 55, is believed to have been instrumental in bringing Ronaldo to Old Trafford in 2003 as well as the club's other recruits from Portuguese football, Anderson and Nani, last summer.

His departure leaves Ferguson needing to find a new assistant manager with the club about to kick off their pre-season preparations at Aberdeen tomorrow. But, perhaps more importantly, it removes a crucial link between United and Ronaldo, who covets a move to Real Madrid.

Queiroz will now manage the winger at international level but whether he is still as committed to dissuading him from moving on remains to be seen. He certainly has his own horror story to tell, having spent one unhappy season as Real boss in 2003-04 before returning to United as Ferguson's number two.

But, after four successful years at Old Trafford, he now feels the time is again right to strike out on his own. "I am very proud and honoured to be able to come back and serve my country," said Queiroz, who will take full responsibility for all aspects of Portugal's national teams, including oversight of the youth teams.

"It is a very special challenge and a position that any professional would want. I also want to thank Manchester United for our established agreement that I could return to Portugal if the opportunity materialised."

FPF president Gilbert Madail expressed his gratitude, saying: "I also want to recognise the co-operation of Manchester United from the beginning of this process."

Queiroz, who is to be presented to the media on Wednesday, added that his first priority will be qualification for the 2010 World Cup in South Africa. Ronaldo described Queiroz as "a perfect choice", telling Portuguese broadcaster RTP: "I have been working with him for some time and I just have to say that he is very good for this job."

Meanwhile, Ferguson will watch United begin their pre-season preparations in the familiar surroundings of Pittodrie still fuming at the sudden explosion of debate into Ronaldo's future.

The intervention of FIFA president Sepp Blatter and his comments about "modern slavery" in the game has put the prospective world player of the year's future back at the top of the footballing agenda. Once again, United find themselves at odds with one of the most skilled politicians in the game.

Although irritation at Ronaldo is obvious, there can hardly be any public stance against the 23-year-old given he is such an important part of Ferguson's plans.

But as he takes his team to Aberdeen for tomorrow's friendly, Ferguson - not for the first time - is bound to reflect on Blatter's ability to gain mileage out of any subject he feels suits his own ends. It was only in February that the Swiss spoke out against any attempt to railroad clubs into selling players against their will.

Yet, it seems a relatively low-profile case involving Andy Webster being allowed to leave Hearts for Wigan did not trigger the same stance Blatter is now adopting over Ronaldo's proposed move to Real Madrid. Webster was able to swap Hearts for the JJB Stadium by buying out the remaining year of his contract after a verdict was reached by the Court of Arbitration of Sport (CAS), upholding his right to do it.

At the time, Blatter commented: "The decision which CAS took on January 30 is very damaging to football and a victory for those players and their agents who are toying with the idea of rescinding their contracts before they have been fulfilled.

"This unfortunate decision undermines the principle of contract stability as agreed with the European Commission in 2001 as part of the new transfer regulations which restored order to the transfer system. This has been deemed less important than the short-term interest of the player involved."

Those comments are clearly at odds with Blatter's stated belief that clubs should not hold players against their will, which offered Ronaldo the platform to speak his mind yet again.

The former Sporting Lisbon player will not be part of the Red Devils squad heading to Scotland ahead of their four-match trip to Africa over the next fortnight. His recent ankle operation will ensure he is not part of Ferguson's plans for as long as three months.

But that does not mean the Madeira-born player will be far away from his manager's thoughts.