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 before having ice taped to his ankle and was visibly distraught as team staff worked on him.
Asked why he had not made the decision earlier, coach Tite said: “I didn’t see Neymar was injured. The capacity he has to overcome it, it tricked me. I only saw it now when I watched the video.”
Team doctor Rodrigo Lasmar was cagey on the prognosis after the match indicating there was “trauma” to the ankle but that they may need to wait 24 to 48 hours to make a full assessment.
Tite, though, remained buoyant about his star man. “We are confident that Neymar will continue playing in the World Cup,” he added.
Defender Danilo also picked up an ankle problem, hobbling through the final minutes.
The five-time winners next face Switzerland on Monday.
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.