Ferguson, who retired as United manager at the end of last season following a trophy-laden stint of nearly 27 years in charge at Old Trafford, oversaw the early years of football icon Beckham's career.
But relations between the pair became increasingly icy with Beckham, who would become England's most-capped outfield player, leaving United in 2003 to join Spanish giants Real Madrid.
Ferguson, in an interview with US television network PBS, a transcript of which was carried on the website of Britain's Times newspaper on Wednesday, said Beckham and his wife saw a change in their "celebrity status" after marrying and expressed astonishment at the player's later decision to join the Los Angeles Galaxy, then little known outside the United States, in 2007.
"As a young kid when I got him at 12 years of age his great desire was (to play) some of the best football," Ferguson said.
"He was a fantastic trainer, practised all the time. At night-time he would come by with the schoolboys and practise with them. And he was in that collection.
"And then of course, his life changed, when he married the girl (Adams) from The Spice Girls, yeah. And his focus changed," the Scot added.
"He got drawn into the that celebrity status, you know And for me, I'm a football man. I'm a football man.
"He lost the focus. And when we sold him to Real Madrid, he did well."
Beckham won one La Liga title in four years with Real before moving to the United States.
"I couldn't believe that he goes to LA Galaxy," said Ferguson. "I couldn't believe that. I couldn't understand that. I would never do that, you know
"If I was going to go, I would be making sure he would go to the best, and Real Madrid was the best, I would say, and United."
However, an admiring Ferguson also said: "But he reinvented himself. He goes and plays for the English national team for a couple more years. He goes and plays for AC Milan (in the) European Cup. He's unbelievable. He really is and good, well done to him. You can't argue with the status he has in life."
Beckham retired from playing football professionally aged 38 at the end of last season following a brief spell with France's Paris St Germain.
Ferguson said he wished Beckham, whose stellar celebrity unlike that of previous globally-known footballers such as Brazil's Pele or Argentina's Diego Maradona -- was far in excess of his ability, had become a truly great player but could not argue with the life he'd made for himself.
"Well, for me, it would have pleased me more to see him be a great, great player, you know" Ferguson added.
"As I said, I'm a football man. But how can I argue with life He's an icon for young people... and represents himself the proper way. And I say well done.
"Amazing boy, yeah. I mean, how he has created himself. He's fantastic. Wonderful boy."