The PSG star appeared to pick up the injury when tackled during a counter-attack in the second-half of the 2-0 win at the Lusail Stadium.
He continued to play on before being removed in the 79th minute.
He appeared to have ice taped to his ankle and was visibly distraught as team staff worked on the knock.
The team doctor was cagey on the prognosis for the Selecao’s star man after the match indicating they may need to wait 24-48 hours to make a full assessment.
Brazil next face Switzerland on Monday.
Neymar is yet to win a World Cup with the Selecao last coming in 2002.
Two goals from Richarlison were enough to secure all three points on Thursday night and joint top of the fledgling Group alongside the Swiss who beat Cameroon earlier in the day.