Brazil and Al-Hilal forward Neymar will have surgery after rupturing his anterior cruciate ligament and meniscus in his left knee while on international duty.
The 31-year-old was taken off on a stretcher in tears during the 2-0 defeat by Uruguay on Tuesday.
He landed awkwardly after a challenge in the first half of the 2026 World Cup qualifier in Montevideo.
"It's a very sad time, the worst," Neymar wrote on Instagram.
"I know I'm strong, but this time I'm going to need my family and friends even more."
It is unclear how long Neymar will be out for but the usual prognosis for recovery from an ACL rupture or tear can be up to eight to 10 months.
"Brazilian and world football need Neymar healthy and recovered, because football is happier when he's on the pitch," Brazilian Football Confederation president Ednaldo Rodrigues said in a statement.
Brazil's all-time male top scorer, who moved to Saudi Arabia in the summer from Paris St-Germain, has struggled with injuries in recent years.
He missed two World Cup games in Qatar last year after he was injured in Brazil's opener against Serbia.
Neymar had also been sidelined for multiple months of his last two seasons with PSG because of different ankle injuries and his final campaign in France was cut short in February by ankle surgery.
"It's not easy to go through injury and surgery, imagine going through it all again after four months of recovery," he added.
At the 2014 World Cup in Brazil, the then 22-year-old broke a bone in his back following a bad challenge by Juan Zuniga in the quarter-final against Colombia.
Brazil won that match but Neymar was ruled out for the remainder of the tournament, with his country going on to lose 7-1 to Germany in the semi-finals.