"At the time he had just come back from an achilles injury and we thought he had just lost a little bit. We got the offer from Lazio, £16.5m for a centre-back who 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," he is quoted by the Independent on Sunday.
Ferguson also spoke of his admiration for Paul Gascoigne, who chose Tottenham over United when he left Newcastle United.
"Gascoigne was quality. I think he was the best young player to come out of England in the last 30 years at that time."