Neymar will miss Brazil's second World Cup match, but will remain with the squad undergoing treatment for his ankle injury, the team doctor said Friday.
Rodrigo Lasmar said tests conducted on his right ankle showed that Neymar has a ligament injury and will not be fit to play against Switzerland on Monday.
"It's important to remain calm, an evaluation will be conducted daily so we can have more information and take the most appropriate decisions," Lasmar said, adding that "the goal is to have him recover" in time to still play in the tournament.
Lasmar did not give a timetable for Neymar's recovery. He said the situation of defender Danilo, who has a left ankle injury, was similar to Neymar's and he would also miss the game against Switzerland.
Neymar was receiving treatment at the team's hotel Friday after getting injured in the 2-0 win against Serbia in Brazil's opener.
The Paris Saint-Germain striker sprained his ankle in the second half of a bruising match against Serbia on Thursday. He was in tears while sitting on the bench with ice around his ankle and limped off the field on his way to the locker room.
"Difficult game, but it was important to win," Neymar said on his social media accounts not long after he left Lusail Stadium without talking to the media. "Congratulations team, first step taken. Six more to go."
In an Instagram story after the match, Neymar posted a text that mentioned the need to "have faith."
"It's about believing that everything will be OK despite the chaos. It's the certainty that the best is yet to come. It's understanding that everything has its own timing," the text said.
Brazil coach Tite had already expressed optimism about Neymar's condition. "You can be sure that Neymar will still play in this World Cup," he said after the match. "I'm certain about that."
Neymar was substituted in the 79th minute, about 10 minutes after hurting his ankle while being tackled by Serbia defender Nikola Milenkovic.
Neymar had already been tackled many times before during the match, and was seen grimacing and limping a few other occasions. He was fouled nine times, the most of all World Cup players after the first round of matches in the group stage.
"We'll remain upbeat, it's the characteristic of this team," Brazil goalkeeper Alisson said. "These adverse circumstances will only make the group stronger."
The five-time world champions end their group-stage campaign against Cameroon on Friday, December 2. Brazil leads Group G along with Switzerland, who opened with a win against Cameroon.
If Neymar can't play, his replacement would likely be Rodrygo, who came in as a second-half substitute against Serbia on Thursday.
The 30-year-old Neymar was also hurt at the 2014 World Cup in Brazil, when his tournament ended with a back injury in the quarter-finals against Colombia. He had to be taken off the field on a stretcher, and Brazil ended up losing to Germany 7-1 in the semi-finals.
Neymar's right foot has caused him problems in the past, being among the series of injuries he suffered since the 2018 World Cup in Russia. Another right ankle sprain led him to miss the 2019 Copa America that Brazil won.
Neymar has yet to win a major title with the national team. He helped the Selecao win the 2013 Confederations Cup and its first Olympic gold medal at the 2016 Rio Games.
With 75 goals for the national team, he is two shy of Pele's scoring record.
Brazil's remaining World Cup group-stage fixtures
Monday, November 28: Brazil vs Switzerland (Stadium 974, Doha; kick-off 4pm)
Friday, December 2: Cameroon vs Brazil (Lusail Iconic Stadium, Lusail; kick-off 7pm)
Analysis: Neymar injury overshadows Brazil win
Sky Sports football features writer Adam Bate:
"This World Cup was supposed to be Neymar's redemption song after two failed tilts at the trophy. This is likely to be his last chance and he came into the tournament in impressive form for Paris Saint-Germain. Now injury could put paid to his plans.
"The sight of him slumped on the bench after being substituted and the images that have emerged of his swollen ankle are alarming. Even with 11 days until Brazil's likely round-of-16 tie, there is little time for Neymar to recover and show his best form.
"Brazil have other attacking options, though most are more suited to the wide positions than the No 10 role that Neymar has made his own. But this is about the individual as much as what Tite does next. A career-defining tournament is in danger of getting away from him."
'Neymar is in the form of his life'
Sky Sports' Micah Richards speaking to the BBC:
"We have talked about Neymar and this being his competition. He's in the form of his life this season and it is a worry when he goes down like that.
"Let's hope it's not too serious - you don't want your main man injured especially in the first game."