Gary Neville has highlighted the sale of Dutch defender Jaap Stam to Lazio in 2001 as Sir Alex Ferguson's biggest transfer mistake during his tenure at Manchester United. Neville, who played alongside Stam, described him as an unbeatable powerhouse akin to Virgil van Dijk at his peak. Despite Ferguson's illustrious career, which included winning 38 trophies and signing iconic players like Cristiano Ronaldo and Wayne Rooney, he admitted in his autobiography that selling Stam was a significant error. The sale reportedly stemmed from a fallout over comments in Stam's autobiography and concerns about his fitness. Ferguson's broader transfer strategy often involved signing the best Premier League players, a philosophy that helped United dominate English football but may be challenging to replicate today. Other notable departures Neville regretted include David Beckham, Roy Keane, and Paul Ince.