1. Gerard Pique
Gerad Pique spent time at the youth academy of Manchester United and broke into the team in 2005 but ended up going to Barcelona after feeling homesick. At that time no one probably knew how successful Pique would go on to be. At Barcelona, he has won countless trophies and is regarded by many as one of the best defenders in the world.
Pique would not have made the team at Manchester United though for quite a while. His time at Manchester United overlapped with that of Rio Ferdinand and Nemanja Vidic. They were one of the best defensive partnerships in the world at that time and so leaving one of them out for a 21-year-old Spaniard would have been a bad decision. It was a decision Sir Alex Ferguson was not willing to take.
Having said that, Manchester United would now love to have a defender like Gerard Pique in their squad as he adds a lot of experience and leadership. While one cannot blame Sir Alex Ferguson for the departure of Gerard Pique but many Manchester United fans still regret selling him to Barcelona.
2. Paul Pogba 
Pogba would go on to become one of the best midfielders in the world at Juventus. Manchester United were very keen to get Pogba back. They eventually got their wish but it was an expensive deal as they had to spend £89 million to reacquire him.
Pogba had joined Manchester United in 2009 and made his Premier League debut in 2012. He did not get much playing time and was eventually sold off to Juventus. Juventus paid a mere £800,000 to acquire his services.
Pogba’s agent Mino Raiola was instrumental in Pogba’s departure from Old Trafford. In Ferguson’s autobiography, he talks about agents he does not like. As you can imagine Mino Raiola, Paul Pogba’s agent finds himself on the list.
3. Cristiano Ronaldo
Another transfer which greatly impacted the football world was when Ronaldo was sold to Real Madrid for a world record fee at that time. £80m seemed like a lot of money then but it is fair to say Real Madrid ended up getting the better deal.
Sir Alex Ferguson paid £12m in 2003 to Sporting Lisbon to acquire the services of Cristiano Ronaldo. This transfer would change the footballing world forever.
Ronaldo has gone on to become one of the best strikers in the world and has won 4 Champions Leagues with Real Madrid along with 4 World Footballer of the Year awards.
Manchester United have won trophies without Ronaldo but it has not been the same. They are still struggling to find a player who is worthy of wearing the number seven shirt which was famously worn by Ronaldo.