Neymar had to be replaced at half-time of Santos' goalless draw against Palmeiras in Wednesday's Brazilian Cup game with a suspected right thigh injury.
The Brazilian forward started the encounter at Vila Belmiro but late in the first half complained of pain in his right hamstring and could barely stay on the pitch until the interval.
On his way down to the changing rooms, Neymar, 34, was seen in tears and did not return after the break.
Neymar will undergo tests later this week to confirm the extent of the injury.
Neymar had started in Santos' last three league games and was included in Wednesday's starting line-up with Santos needing to overturn a 3-0 first leg deficit to reach the Copa do Brasil semifinals.
"We'll have to manage without him," Santos coach Cuca said regarding Neymar after the game.
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Santos, who are fighting to avoid relegation, also lost winger Álvaro Barreal and defender Lucas Veríssimo to injuries against Palmeiras.
"We've lost Neymar, we've lost Barreal, we've lost Lucas, and we have to pull together as a team now," Cuca said.
"Let's hope the injuries aren't serious and that they can return as soon as possible."
Neymar has scored eight goals in 24 appearances this season. Santos are 14th in the league standings, four points clear of the relegation zone.
He was critical of the groundskeeping crew in a post he made on his Instagram stories, saying "I would like to congratulate our stadium grounds manager at Vila Belmiro... Yesterday the grounds were in the worst shape ever seen, full of holes because they decompacted the pitch 1 day before the game. UNBELIEVABLE!!! In addition to playing against an opponent, we also had to play against the pitch...and on top of that we had to use more force to start and stop because the pitch is so irregular. It was not for nothing that 3 fell in the first half... CONGRATS... GREAT WORK of M£rD@"
