The Dutchman was sold to Lazio in a deal worth £15.3m despite being one of United's most consistent players. In his autobiography, serialised in the Sunday Mirror, Stam revealed Ferguson forced him out of the club as United needed the money.
There were injury concerns over the centre-back, but the then-29-year-old defender went on to play five successful seasons in Italy before returning to Holland with Ajax.
And Ferguson has admitted in an interview with MUTV that he still regrets losing the imposing centre-back.
"When I think of disappointments, obviously Jaap Stam was always a disappointment to me, I made a bad decision there," said Ferguson as reported by the Daily Mail.
The legendary Old Trafford boss has had more than a few transfer hits and misses over the course of his career, but the Stam sale still rankles.
Transfer flops under Ferguson included Kleberson and Eric Djemba Djemba, as well as Juan Sebastian Veron and Diego Forlan. But the hits include Cristiano Ronaldo, Wayne Rooney and Ruud van Nistelrooy.
In just three years at Old Trafford, Stam won three Premier League titles, one Champions League and an FA Cup.