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 year 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.
"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.
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"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.
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.