Ahmed Gaballah, head of Ahly's medical department, said Belammari underwent MRI scans that confirmed the tear.
"The player is undergoing tests and an MRI scan, which confirmed a tear in the knee's cruciate ligament," Gaballah told the club's official website. A date for surgery will be set in the coming period, he added.
Belammari picked up the injury during Ahly's 3-0 pre-season friendly win over Spain's Badalona, having joined the club on a three-and-a-half-year deal in January. He has made 19 appearances for Ahly across all competitions since arriving.
In the Badalona win, fellow new arrivals from Enppi, Ali Mahmoud and Aqtay Abdallah, scored either side of a goal from Ahmed Zizo, while striker Sofiane Bendjeddida had a first-half penalty saved.
Ahly's training camp in Spain runs until 20 August and includes further friendlies, among them a meeting with Barcelona in the Joan Gamper Trophy on 19 August.
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