#2 Luciano Vietto
Luciano Vietto was thought to be the next big thing when he arrived at Atletico Madrid from Villarreal for £18m back in 2015. He would spend a season on loan at Sevilla shortly after but failed to really make much of an impact, which was the beginning of the end for the striker.
A brief six-month loan at Valencia was meant to prove he still had it, but in reality, it just exacerbated the worries about his declining ability. He went from having a market value of around £15 to a mere £5m within two seasons - and he was only 24 years old at the time.
His latest loan spell with Fulham was probably the last-ditch effort from Atletico to see whether Vietto was as good as they first thought, but the London club would be relegated from the Premier League. Vietto is to return to Atletico at the end of June, but whether he'll stay there now is unlikely.