A fractured metatarsal curtailed the Brazilian's last season, and the recurrence comes just three weeks ahead of the first leg of PSG's crunch Champions League clash against Manchester United.
The severity of the new injury is not yet known, although Neymar was able to walk off the field unsupported.
“Initial tests carried out revealed a painful reactivation of the injury to the fifth metatarsal,” a club statement said Thursday. “The treatment will depend on progress over the coming days,” it said, adding all medical options would be considered.
Neymar initially tried to continue playing. But he covered his eyes - in tears, according to witnesses - as he was forced off to be replaced by Moussa Diaby just after the hour mark.
His early departure was a worrying moment for PSG, with the world's most expensive player having missed the final three months of last season after breaking the same bone.
Each season PSG, which dominates France's domestic league, is judged according to its Champions League results.
PSG coach Thomas Tuchel after the game said: “The doctor is worried, 'Ney' is worried, because it is the same foot, the same place. It was a situation where he was fouled three times, one after the other. The referee didn't give anything. He twisted his foot.”
The PSG coach was also unhappy with suggestions from the Strasbourg players and coach that Neymar had provoked the injury by winding up his opponents.
“It's Neymar's style, but don't come and complain when you get kicked,” Strasbourg's Anthony Goncalves told broadcaster Eurosport.
“He is a great player, I respect him. He can enjoy himself but don't come blubbering afterwards.”
Edinson Cavani and Angel di Maria got the goals on a bitterly cold night at the Parc des Princes.
Tuchel had left out the ill Kylian Mbappe for this game, with Eric Maxim Choupo-Moting handed a rare start in place of the superstar France forward.
The Cameroon international set up Cavani for the fourth-minute opener, before Di Maria - who had earlier hit the post - finished into the roof of the net 10 minutes from time as PSG stay on track to win the French Cup for the fifth season running.
Before kick-off, fans applauded and chanted the name of Emiliano Sala, the former Nantes striker who is feared dead after the plane he was travelling in disappeared over the Channel Islands on Monday as he was on his way to join his new club Cardiff City.
Nantes' own Cup tie away at third-tier Entente Sannois Saint-Gratien has been postponed until Sunday, with their players still in shock over Sala's disappearance.
Elsewhere, Caen ended the run of plucky sixth-tier amateurs Viry-Chatillon in a 6-0 rout, with Yacine Bammou setting the Ligue 1 club on their way.
An emotional Bammou raised his hands towards the sky after his early goal, in tribute to Sala, with whom he played at Nantes last season.
Caen coach Fabien Mercadal said Bammou had made clear his desire to be involved despite the tragic turn of events.
“We talked about it. What happened is not fair. He was a friend to Yacine, he was deeply affected. The worst thing for him would have been to not play,” said Mercadal.
A group of Caen supporters, for whom Sala spent the second half of the 2014-15 season on loan, also fastened a banner that read “Emiliano” to a fence surrounding the pitch.
Naim Sliti scored a hat-trick as relegation-threatened Dijon claimed a stunning 6-3 win away at Saint-Etienne for their first victory under new coach Antoine Kombouare, while Rennes overcame fourth-division Saint-Pryve 2-0.
Lyon go to Amiens on Thursday, while Thierry Henry's Monaco were knocked out on Tuesday when they slumped to a 3-1 home defeat against Metz of Ligue 2.