The Brazil international left the pitch in tears, and a PSG statement said he had suffered a recurrence of the metatarsal injury that threatened his World Cup hopes last year.
"Initial examinations have revealed a painful recurrence of his injury to the fifth metatarsal of his right foot," the statement said.
"The treatment of this injury will depend on how it evolves over the next few days. All options can be envisaged at this time."
Speaking after the match, coach Thomas Tuchel said: "Neymar is worried because it is the same foot, his right foot, in the same area. If Neymar asks to come off, it is always something complicated."
PSG medical sources indicated that the Italy international was unlikely to feature again until the last match before the trip to United, against Bordeaux in Ligue 1 at the Parc des Princes.
Tuchel is desperate for midfield reinforcements, with pressure on sporting director Antero Henrique to bring in new faces after Ajax's Frenkie de Jong chose Barcelona over PSG.