The Paris Saint-Germain superstar is set to undergo an operation this coming weekend, despite his coach Unai Emery initially being hopeful that the 25-year-old would play a part in next week's Champions League clash against Real Madrid.
Surgery will take place Saturday, national team surgeon Rodrigo Lasmar said.
The lay-off means Neymar may miss the rest of the season with PSG, returning to fitness just weeks before Brazil's World Cup campaign.
"I'm not a doctor, so we have to wait for the club's decision", he said.
Neymar missed PSG's 3-0 win over Olympique Marseille in the French Cup on Wednesday night but he later posted a message on Instagram: "Obstacles should not impede you", he wrote alongside a picture taken moments after he was injured.
The attacker, who became the most expensive footballer in history with last summer's transfer from Barcelona, now looks a major doubt to play another game for PSG this season. "We have lost our best player, but we have other fantastic players and we will need everyone in order to beat Real Madrid". But they are still the champions of Europe.
PSG won the match 3-0, but Neymar's injury marred the contest, as he rode a challenge from Bouna Sarr and twisted his ankle upon contact with the turf, while also suffering a hairline fracture to his foot.
Newspapers featured close-up illustrations detailing Neymar's foot and ankle.
"Above all we're sad for him as a man, for Ney, because he is someone who really loves football and who loves playing in this kind of match", offered midfielder Marco Verratti. "But there is a noticeable haste to find a solution so that Neymar will have recovered in time", an ESPN Brasil sports commentator, Mauro Cezar Pereira, told AFP.