Manchester United are one of the biggest clubs in the world thanks to Sir Alex Ferguson, who shaped a new era at Old Trafford.
He was known for making statement signings in the transfer window, and he got it right more often than not. Some of the best bargain signings of his era have included greats like Eric Cantona, Cristiano Ronaldo, Ole Gunnar Solskjaer and Edwin van der Sar.
When it comes to selling players, Sir Alex sold many big names, and some of these turned out to be poor decisions. Managers sometimes misjudge players' abilities, and even Ferguson made a few mistakes during his managerial tenure at United.
On that note, here are five players who should not have been let go of by the legendary Scotsman.
#5 Jaap Stam
Jaap Stam was one of the most successful defenders of the Premier League era. He was an integral part of the treble-winning United side, and he finished each of his three seasons with United as a Premier League winner.
At 6'3", he was quick, physically strong, excellent in the air and an intimidating figure in United's backline. The centre-back was destined for greatness at Old Trafford, but his career in Manchester ended somewhat prematurely. He was forced to leave the club by former United boss Sir Alex Ferguson after a misunderstanding between the duo.
Stam moved to Lazio in a £16.5 million deal in 2001. He then went on to achieve great things in Italy and was an integral part of the Milan side that lost to Liverpool in the famous 2005 UEFA Champions League final at Istanbul.
He had a brief stint with Ajax, before retiring in 2007 as one of the best centre-backs in the world. Ferguson later admitted that allowing Stam to leave United was one of the worst decisions he had ever made.
#4 Gerard Pique
When you have Nemanja Vidic and Rio Ferdinand playing at the back, there is no way a 21-year-old centre-back could break that partnership, no matter how good he plays. This was the story of Gerard Pique at Manchester United. He was one of the most exciting young defenders in the Premier League when he made his United debut in 2004.
He made his full debut for United in 2006 but was never able to break into the first team consistently. He eventually moved from United to his boyhood club Barcelona in 2008 and went on to become one of the best centre-backs in the world.
He was an integral part of the Barcelona team that has dominated the world over the years and is now in the twilight of his career. He also won the FIFA World Cup with Spain in 2010 and the European Championship in 2012.
Pique's Old Trafford exit was his choice and not anyone's mistake, but still, many United fans regret that they let go of him without a fight.
#3 David Beckham
David Beckham joined Manchester United as a 16-year-old with a lot of potential and became one of the best midfielders in Europe. He made his Manchester United debut aged 17. He was with United for 12 long years before making his move to Real Madrid in 2003.
Beckham had a hugely successful stint with Manchester United, winning six Premier League titles and one UEFA Champions League title. He made 394 appearances for United and scored 85 goals.
During his Old Trafford days, he was renowned as one of the best set-piece experts in Europe. He possessed a fantastic right foot, and his free kicks were a joy to watch.
Beckham had a breakdown in his relationship with Sir Alex Ferguson before finally moving to Real Madrid. He had four successful years with Real Madrid where we won the La Liga title in 2007. He then moved to LA Galaxy during the summer of 2007.
Throughout his playing career, Beckham became known as one of the most famous and charismatic players in world football. United struggled in his absence until they found a gem in Cristiano Ronaldo.
#2 Paul Pogba
Paul Pogba was an expensive mistake made by Sir Alex Ferguson. Manchester United sold him for £800,000 in compensation to Juventus when the midfielder left Old Trafford in 2012.
Pogba joined United in 2009 and made his Premier League debut in 2012. He played three times for the club and moved to Juventus in the summer of 2012, after refusing to sign a contract extension with the Red Devils.
Pogba's agent Mino Raiola played a significant role in his transfer saga, and it left Ferguson furious. He even mentioned him in his book, saying:
"There are one or two football agents I simply do not like and Mino Raiola, Paul Pogba's agent, is one of them"
It turned out to be an extremely costly mistake when Manchester United paid €110 million for the same player in 2016, after letting him go for a six-figure sum four years earlier.
#1 Cristiano Ronaldo
If there is a transfer that no United fan will forget, this will be the one. Cristiano Ronaldo left United for Real Madrid in the summer of 2009 after spending six successful years at Old Trafford. This was one of the most regrettable moments for everyone at Manchester United.
The Red Devils paid £12 million for Ronaldo in 2003 when he was just 18 years old. He matured under Ferguson's tutelage and went on to win three Premier League titles and one UEFA Champions League title.
He established himself as one of the best wingers in world football and at 24, had the world at his feet as he claimed his first Ballon d'Or award.
He had much bigger dreams, and Ferguson could not stand in his way, as he joined Real Madrid in a record-breaking deal worth £80 million. He has gone on to achieve a lot more at the Santiago Bernabeu, winning four UEFA Champions League titles.
Ronaldo has since then played for Juventus and is now back at Manchester United following his transfer last summer. The 37-year-old is also the leading goalscorer in men's international football with 115 goals to his name.