Mbonambi will be out of action for two weeks after a small ligament tear to the knee, team doctor Org Strauss confirmed.
Prop Morne Mellett has been declared fit, following a neck spasm and will train on Monday, as will winger Travis Ismaiel.
Grant Hattingh, Francois Venter and Ulrich Beyers will not train Monday due to minor ailments, but will be fit for selection.
Meanwhile, Springbok winger Bjorn Basson has been released for Currie Cup duty this weekend.
Jean Cook (thumb, two weeks), Callie Visagie (ankle, three weeks) and Courtnall Skosan (quad, four weeks), could all still play in the competition later in the season.
The long-term injury list is: Basil Short (knee, four months), Francois Hougaard (ankle, four months), Cornell Hess (knee, five weeks), Deon Stegmann (shoulder, four months), Arno Botha (knee, three months), Pierre Spies (biceps, three months) and Louis Fouche (knee, three months).