4. Diego Forlan
Before Uruguayan legend Diego Forlan was a Manchester United player, the striker could have been playing for another team in Red, in the form of Middlesbrough. In 2003, Boro had agreed a fee with Forlan’s then-club, Independiente. Due to disagreements over how the fee was going to be paid, this allowed United to show their financial prowess and offered Independiente the money upfront, something the Riverside outfit couldn’t match. Forlan became a Red Devil, but didn’t show the attacking power that he produced in the latter stages of his career.