Ferguson sold Stam to Lazio shortly after the Dutch defender?s controversial autobiography was published in 2001.
Now Sir Alex has confessed that the £16.5million sale to the Rome club was a mistake "in football terms." But he said, at the time, the offer was too good to turn down.
Everyone assumed the book was the reason behind his departure. But Ferguson maintains his only criteria were Stam's inability to regain top form following injury, and the tempting offer from the Serie A outfit.
"At the time he had just come back from an Achilles injury and we thought he had just lost a little bit," he told an audience at the Citizens Theatre in Glasgow.
"We got the offer from Lazio - £16.5million for a centre-back that was 29. It was an offer I couldn't refuse.
"But in playing terms it was a mistake. He is still playing for Ajax at a really good level."