Morocco start their Group C campaign against DR Congo on Monday, then face Togo on 20 January and take on Ivory Coast four days later.
Rherras, 23, and Cameroon midfielder Arnaud Djoum, 27, will miss Hearts' Scottish Cup meeting with Raith Rovers on 22 January.
Cameroon are in Group A with Burkina Faso, Guinea-Bissau and hosts Gabon.
Djoum has five international caps while Rherras made his Morocco debut in August.
Scottish Premiership clubs are currently on their winter break and Hearts' next league fixture is away to Celtic on 29 January.
Hearts left-back Faycal Rherras has been given a late call-up to Morocco's Africa Cup of Nations squad.