It wasn't immediately known how serious Neymar's injury was following the Serbia game. It appeared worse than a Grade 1 sprain based on the swelling that was evident as soon as he left the field. Neymar would not have been able to play for the rest of the World Cup if he had sustained a Grade 3 sprain (which would be a complete tear) or a more serious Grade 2 sprain (which would be a significant partial tear).
Neymar did end up skipping the final two Group Stage matches. Brazil also appeared substantially less successful in both games against Switzerland and then Cameroon without Neymar. With Neymar already back on the practice field, it appears that he most likely suffered from a Grade 2 sprain, which wasn't particularly very serious.