He did have good potential in real life, believe he was best young player at Copa America the season Liverpool signed him and was being tipped for good things but had a series of injuries and **** loan spells...so maybe he got over the injuries and reached that potential?
The pressure in the Portuguese league is way lower than in the English one. maybe he just needed a lower league to claim his potential.
In Portugal he will get, with ECL included, max 8 games in a season where he can't make mistakes. In Sunderland we wull have, without ECL, 20?