Neymar is smart and has great skills so it's not problems for him to return to training and playing at not top-notch level. His main barrier is lack of self-control on his eating habit and over party participation that cause some issues on hit fitness. It will affect his performances on field and potentially cause him injuries. Neymar knows that he is not allowed to stay in Brazil, enjoy parties while Al Hilal are paying him very high salary. They need Neymar on field, in matches, not in parties and if things won't improve, they will terminate the contract and find another star player.
regarding weight gain, I think it could happen and before he returns to the field he will definitely do training and with this training, Neymar will lose weight so he can be physically stronger.
but the question, has the injury really healed?
I'm just worried that in the near future Neymar will return to the field to make a contribution to Al Hilal, he will experience injury problems again and if that really happens, I'm sure Neymar chance will have run out and Hilal will do the same thing as Ittihad, who loaned his star players to other teams.