Ben Collett, now 23, was at the High Court in London yesterday to hear a judge award him more than 4.3 million in damages, which his lawyers said later was a record, including 3,854,328 for future loss of earnings.
But Mrs Justice Swift said that, when future loss of pension and interest on past loss of earnings were determined, the final pay-out was unlikely to be less than "four and a half million".
Sir Alex told the judge at a recent hearing in Manchester that Mr Collett had an "outstanding chance" of becoming a full-time professional, but, at age 18, his right leg was broken in two places in a tackle by Gary Smith, of Middlesbrough FC.
Mr Collett joined United's youth academy aged nine and signed youth contracts, becoming part of the club's FA Youth Cup-winning team in 2003.
But in his first game in the reserves, in May 2003, Gary Smith's tackle brought his career to a premature end.
After the ruling, Mr Collett's solicitor, Jan Levinson, of law firm Beachcroft, said: "Ben Collett and his family are happy this case has finally come to a close and that Ben is able to move on to the next chapter in his life."
He said: "The size of the award reflects his talent and potential, prior to the tackle, as one of the brightest young footballers in the country.
"Having said that, Ben would, understandably, have preferred to earn this sum through a full career as a professional footballer."
Mr Levinson said: "The judgment announced today is the highest award ever given to a professional sportsman or sportswoman."
Mr Smith and Middlesbourgh FC had admitted liability for the "negligent" tackle, and the judge's task was to decide on the level of compensation Mr Collett should receive from the club's insurers.
Mrs Justice Swift said she had found that, had it not been for the injury, Mr Collett would have been offered a three-year professional contract with Manchester United in July 2003.
Sir Alex Ferguson told Manchester High Court during the hearing: "The boy showed fantastic focus and a great attitude to work hard; they are qualities to give any player an outstanding chance in the game."
As well as evidence from Sir Alex, Mrs Justice Swift heard from other high-profile witnesses, including United's captain, Gary Neville.
Describing the accident, Mrs Justice Swift said Mr Collett was playing for United's reserve team in a match against Middlesbrough's reserves. She said: "In the course of the game he was tackled by the first defendant (Gary Smith).
"The tackle was high and, as a consequence, the claimant suffered a fracture of the tibia and fibula of his right leg."
The evidence was that, "at the time he sustained his injury, he was on course for a successful career in professional football".
Mrs Justice Swift found that Mr Collett "must have had a good chance of spending at least part of his career – possibly the majority – in the Premier League".
She added: "His positive attitude towards his injury and to the devastating blow of being unable to pursue his chosen career does him great credit. I found him a most impressive young man."
Mr Collett plans to take up a place at Leeds University next month to study English.