Gerard Pique
The young Spaniard arrived at Old Trafford as a 17-year-old from Barcelona on a free transfer. Pique made very few appearances for the Manchester United first team and was mostly used in the Reserves.
Following a successful spell in his homeland at Real Zaragoza, Pique returned to Barcelona in 2007 where he became a part of the squad that won the Champions League in 2008. In the same year as the Champions League triumph, Pique re-signed for Barcelona for a fee of £5 million and has since become a important part of the squad.
The 26-year-old played a huge part for his country in their 2010 World Cup and 2012 Euro victories and has been touted as one of the best center back in the world. Manchester United could have certainly benefited from his services had he stayed on at Old Trafford.