Both men were on target in Egypt's 3-0 victory over Niger on Friday, with Salah scoring twice either side of a goal from Mohamed.
"The technical staff of the Egyptian national team preferred to rest Mohamed Salah and Mostafa Mohamed for the Liberia friendly after both took part in the previous match against Niger," the EFA said in a brief statement.
Zamalek's left-back Ahmed Fattouh, who came on as a second-half substitute against Niger, will also miss the Liberia game due to a hamstring injury.
Egypt host Liberia at Alexandria's Borg El-Arab Stadium on Tuesday. They will also face Belgium in a high-profile friendly in November.
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