Manchester United boss Sir Alex Ferguson has admitted he was too quick to write off defender Jaap Stam. Ferguson sold the Dutchman to Lazio in a move viewed by many United fans as one of his few errors during more than two decades in charge at Old Trafford. And while Ferguson still insists the £16.5million offer from the Rome club was too good to refuse, he has confessed the sale of Stam was a mistake 'in football terms'. The Holland international's exit from Old Trafford in August 2001 came shortly after his autobiography was published amid controversy. He claimed that Ferguson had illegally tapped him up before buying him from PSV Eindhoven and revealed blunt insights from the United dressing room. Many assumed his book was the reason behind his departure. Stam had won three Premier League winner's medals and the Champions League in his three full seasons in England. 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,' the Scot 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.' Stam went on to shine for Lazio and they sold him to AC Milan for around £7million in 2004 after he rejected a new contract. And the 35-year-old - who previously claimed United knew they had made a mistake in selling him - signed a two-year deal with Ajax in summer 2006. Ironically, Lazio funded the purchase of Stam through the £28.1million sale of Juan Sebastian Veron to United the previous month. Veron failed to live up to his price tag at United, who replaced Stam with Laurent Blanc, the French defender who was approaching his 36th birthday. The Red Devils failed to win a trophy in that 2001-02 season, which was meant to be Ferguson's last in charge before he performed a U-turn on his scheduled retirement.