The Brazil international rolled on the floor in visible pain following a tackle from a Saint-Etienne defender, leading him to be stretchered off on 85 minutes as teammate Lionel Messi inspired a come-from-behind victory.
Neymar took to Instagram after the game and promised to come back stronger from the potentially-lengthy injury.
"Unfortunately, these setbacks are a part of an athlete's life," he said on Instagram.
"Now you have to raise your head and move on. I will come back better and stronger."
Neymar is stretchered off in the final minutes against St-Etienne in what appears to be an ankle injury 😞 pic.twitter.com/P8Jq4bxIKM— Football on BT Sport (@btsportfootball) November 28, 2021
PSG boss Mauricio Pochettino said it was not yet clear the extent of the injury, adding the club will know more in the coming days.
"For the moment there is nothing to say," he said in a postmatch news conference.
"He felt pain from the twist of his ankle. Tomorrow, he will take exams and we will know a little more about his injury."
Sunday's result extended PSG's lead at the top of Ligue 1 to 12 points.