Sir Alex Ferguson is arguably one of the best managers in the history of football. There are not many managers who can give the Scottish manager a strong fight when it comes to achievements in the Premier League.Alex Ferguson took Manchester United to greatness when most people had written him off. He gave the Red Devils recognition on the biggest stage of football. During his reign at United, Alex Ferguson won 38 trophies, winning the Premier League 13 times, the FA Cup five times, and the UEFA Champions League twice.Sir Alex Ferguson let go of some quality United playersIn his 26 years as Manchester United's manager, the Scotsman scarcely made any mistakes. Alex Ferguson was successful because of his ruthless and fearless approach. His man-management skills are fabulous and that has been reflected in how the former players speak highly about him.That being said, there are a few players who could have done more at Manchester United and helped Alex Ferguson and the team. Due to one reason or the other, they were allowed to leave. Here, we take a look at five such superstars sold by Alex Ferguson:#5 Ruud van NistelrooyManchester United v Manchester CityRuud Van Nistelrooy was a sensation at PSV Eindhoven since he joined them in 1998. In his first two seasons with the PSV, the Dutch striker scored 73 goals in 78 appearances in all competitions for the club.This attracted interest from Manchester United and Alex Ferguson. Because of Van Nistelrooy's injury problems, the transfer deal was delayed by a year and the Dutchman joined United in the 2001-02 season.The former PSV striker was a huge success with the Red Devils as he scored 95 goals in 150 appearances in the Premier League. In his five seasons at the club, Van Nistelrooy won the Premier League, FA Cup, League Cup and the Community Shield.During the 2005-06 season, despite finishing as United's second-highest league scorer with 21 goals, Van Nistelrooy was benched in the League Cup final against Charlton Athletic. He was even benched for United's final league game which added more fuel to the speculation of a rift between the Dutchman and Alex Ferguson. Over time, things escalated and in the summer of 2006, it was confirmed that Van Nistelrooy would be joining Real Madrid.ً@TheFergusonWayGentle reminder of Ruud van Nistelrooy's stats in 2002/03:• Played – 52• Goals – 44• Assists – 6• Premier League Golden Boot• Champions League Golden Boot• FA Cup Golden Boot• Premier League POTS• UEFA Club Forward of the Year• UEFA TOTY• Sir Matt Busby POTY8:14 PM · Mar 24, 20217083847Gentle reminder of Ruud van Nistelrooy's stats in 2002/03:• Played – 52• Goals – 44• Assists – 6• Premier League Golden Boot• Champions League Golden Boot• FA Cup Golden Boot• Premier League POTS• UEFA Club Forward of the Year• UEFA TOTY• Sir Matt Busby POTY https://t.co/dEvsKkgZkJIn his very first season in La Liga, the Dutch striker scored 25 goals in 37 appearances. Being the goal-scoring machine he was, letting him go probably felt like not the right decision by Alex Ferguson. Given the success Manchester United had after Van Nistelrooy's departure, it probably justifies the sale though.#4 David BeckhamDavid Beckham of Manchester UtdThe English midfielder was a product of the Manchester United academy, coming from the elite lot of the Class of 92 players. There is no doubt that David Beckham is arguably one of the best free-kick takers in the history of the game.Alex Ferguson was quoted as saying that the Englishman practiced his set-pieces with a discipline to achieve an accuracy which most players wouldn't care about. Beckham won six Premier League titles, two FA Cups, the UEFA Champions League once and many other accolades during his time at Manchester United.During his nine seasons with the Red Devils, the English midfielder played 264 matches, scoring 62 goals and registering 81 assists. His presence on the right flank gave great problems to the opposition with his impeccable crosses and superb vision. He was a vital player back then for Manchester United.Premier League@premierleagueDAVID BECKHAM AT 40:265 matches80 assists62 goals6 titles15 free-kick goals0 red cards(#BPL stats only) http://t.co/q9Mh5xsgjC1:35 PM · May 2, 201516952456DAVID BECKHAM AT 40:265 matches80 assists62 goals6 titles15 free-kick goals0 red cards(#BPL stats only) http://t.co/q9Mh5xsgjCThe relationship between Alex Ferguson and David Beckham has deteriorated ever since the latter's marriage to Victoria Beckham. The Scotsman was of the opinion that the Englishman's commitment towards the club and his team-mates had drastically changed. Beckham was highly active in the entertainment industry with his personality and eye-catching looks.Over time, things only worsened between the two, especially after the alleged boot incident. It was rumored that after the FA Cup defeat to Arsenal in the 2002-03 season, Alex Ferguson threw a boot at Beckham out of anger and frustration. The boot hit the midfielder over the eye, for which stitches were required to heal the cut. Ever since then, Beckham was linked with a move away from Old Trafford.In the summer of 2003, the English midfielder signed for Real Madrid. Although Beckham never achieved the same success he did at Manchester United, a world-class player was let go by Sir Alex Ferguson. This transfer was the ideal representation of the phrase which states that no player is bigger than the club.