"Unfortunately, the medical examinations have confirmed that Amadou has suffered a rupture of the anterior cruciate ligament," Belgium team doctor Brahim Hacene said in a statement.
"This is devastating news, both for him personally and for the team."
The statement added that, having consulted with his club Aston Villa, Onana will remain with the Belgium squad until at least after their quarter-final game against Spain on Friday.
Onana was replaced in the 21st minute of Belgium's 4-1 win over the U.S. after landing awkwardly, though he celebrated with his teammates on the pitch while using crutches after the game.
FIFA World Cup 2026
The 2026 World Cup is taking place from June 11th to July 19th in the United States, Canada and Mexico. The tournament features 48 national teams and is played at 16 modern stadiums.
Match schedule and times | Group standings and knockout bracket | How to watch the World Cup | World Cup Format | Past winners of the tournament