Former United boss Ferguson oversaw the early years of of the man who would go on to become England's most-capped outfield player but also presided over the midfielder's departure from Old Trafford in 2003.
In an interview with US TV network PBS, Ferguson said Beckham and wife Victoria were subject to a change in their "celebrity status" after marrying and expressed surprise at his decision to join the Los Angeles Galaxy 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 in a transcript of the PBS interview published by The Times.
"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 from The Spice Girls, yeah. And his focus changed.
"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 in Madrid before moving to the United States for a new chapter in his career.
Ferguson added: "I couldn't believe that he goes to LA Galaxy. I couldn't believe that. I couldn't understand that. I would never do that, you know?