#2 Gerard Pique
It's tempting to imagine how well off Manchester United would have been had Sir Alex Ferguson gave Gerard Pique more of a chance during his time at the club. The tall Spaniard made only 23 appearances in total but his path to the first team was blocked by the formidable pairing of Rio Ferdinand and Nemanja Vidic.
At his tender age, there was little chance of Pique splitting up the two but Manchester United could certainly have done with his talents once those two departed.
Pique, instead, returned at the age of 21 to the team of his youth, Barcelona, and has not looked back since: he has 6 league titles to his name; 5 Copa del Rey titles; He also was a rock in defence, as Barcelona triumphed in the UEFA Champions League in 2009, 2011, and 2015.
The Spaniard has established himself as one of the world's best defenders both at the club and international levels, and has developed into a world-class performer. Similar to when Paul Pogba left the club for Juventus due to consistently getting overlooked by Ferguson, Pique's gain was Manchester United's loss.