5. Owen Hargreaves
Bought for: £17m by Manchester United in 2007
Sold for: Released on a free in 2011
Manchester United boss Ferguson features again after taking an ill-advised gamble in 2011, landing injury-prone England international Owen Hargreaves from Bayern Munich for a bumper £17m.
When fit Hargreaves was hugely impressive but those moments proved few and far between, with the versatile midfielder making just 27 Premier League appearances across four seasons at Old Trafford.
After sticking by Hargreaves though multiple rehabs and failed comebacks United eventually severed ties in 2011, only to see their former star sign for arch rivals Manchester City on a free the same summer after bidding to prove his fitness with an extensive workout video posted on YouTube.