I would say that the risk here is bigger.
Yes he is proving to be quality RB specially in attack. He can go forward very well and participate in attack. He is very good at showing up at the end of the plays and the fact that he can score is really good.
The thing is, he still needs to improve his deffending skills. This isn't much noticed in the portuguese league but I really think that it may be noticed in a more competitive league. Despite this, I really think this kid can improve alot and add some skills to his game.
It's too soon to say that he may or not become a world class player. Don't want to risk with an answer. Has the potential to do so.
I would definately say that he should bulk up but that won't be a problem. Just let him train with Valencia at the gym.