The forward suffered an ankle injury during the win against Serbia
Brazil began their World Cup campaign in Qatar with a victory against Serbia which included an early contender for goal of the tournament from Richarlison, but were left with questions over the fitness of Neymar after he sustained an ankle injury.
Chasing a sixth World Cup title, Brazil were particularly impressive for the second half of their 2-0 win against the Serbs at the Lusail Stadium on Thursday night.
Two goals from Tottenham Hotspur forward Richarlison - including an acrobatic second strike - were enough to give them the points, although Neymar's exit 11 minutes from full time took some of the shine off the result.
The star was seen hobbling after a tackle from Serbia's Nikola Milenkovic, and appeared emotional as he was consoled by teammates on the bench.
Images also showed swelling to Neymar's ankle, fueling fears over his fitness for the remainder of the tournament.
Brazil team doctor Rodrigo Lasmar said after the match that it was too early to assess the severity of the injury.
"He continues with the physio, but now need to wait 24 to 48 hours for a better assessment. There is no MRI scan scheduled, but tomorrow we will make a new assessment. We need to wait and cannot make any premature comments," Lasmar was quoted as saying by ESPN.
Neymar, 30, is targeting a first World Cup triumph with Brazil, and is just two goals shy of Pele's all-time goalscoring record for the Selecao.
He has suffered injury agony at a World Cup before, famously missing the semifinal on home soil in 2014 after fracturing a vertebra during the game against Colombia.
Brazil manager Tite, however, insisted that the Paris Saint-Germain forward would play again in Qatar.
"You can be sure that Neymar will play the World Cup. I am absolutely sure about that. He will play in the World Cup," Tite told the media.
Brazil are next in action on Monday in Group G against Switzerland, who opened their campaign with a 1-0 win against Ghana.
Serbia and Ghana will be chasing their first points in Qatar when they meet at the Al Janoub Stadium in Al Wakrah, also on Monday.