Young players who flopped once but turned out to be great footballers afterward:

Patrick Vieira - Milan
Patrick Kluivert - Milan
Thierry Henry - Juventus
Dennis Bergkamp - Inter
Diego Forlan - Man United
Deco - Benfica
Ze Roberto - Real Madrid

Any other examples?

Great shouts from my followers:

Salah - Chelsea
De Bruyne - Chelsea
Lukaku - Chelsea
Aubameyang - Milan
Coutinho - Inter
Cambiasso - Real Madrid
Luis Alberto - Liverpool
Pirlo - Inter

Keep them coming! This is gonna be an interesting thread!