National team officials confirmed that Timber, 25, sustained a mild concussion following a collision in Thursday's practice.
Medical staff monitored the player immediately after the incident and decided he would be unavailable for selection.
"The moment Quinten had the ball, I ran deep," teammate Teun Koopmeiners told Dutch media. "He wanted to chip it to me and then it happened. It wasn't a super hard collision, as I understand it, but you do have to be careful."
The setback for Timber, who celebrated his 25th birthday on Wednesday and was a substitute in the Netherlands' opening 2-2 draw against Japan. His twin brother, Jurrien, has already been forced to miss the tournament with a groin injury.
Having joined the Dutch preparations late following Arsenal's defeat by Paris Saint-Germain in the Champions League final, the defender was forced to drop out of the squad shortly before a warm-up friendly against Uzbekistan in New York.
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.
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