Beckham rose through the ranks at Old Trafford under the guidance of Ferguson and he went on to make 394 appearances for the senior side before his departure to Real Madrid in 2003.
However, Ferguson believes that the 38-year-old's wedding to the former Spice Girl back in 1999 resulted in him taking his eye off the ball.
"His life changed when he married and his focus changed. Well, he got drawn into that celebrity status, you know. For me I'm a football man," the Scot said in an American television interview with PBS.
When asked what he had said to Beckham at the time about his beliefs, Ferguson added: "I just think he's over his head. I don't think he could listen. I always remember in the he says 'I'm in love' and there was nothing you can do with that - therefore he lost the focus. But I saw them in Real Madrid and he did well."
Meanwhile, Ferguson, who retired in May, has also revealed that he has no intention of returning to football management.