Mapimpi is expected to be out of action for four to six weeks.
Am will join the squad in Toulon this week, having returned to training on his rehabilitation from a knee injury suffered in the 24-13 victory win over Argentina in Buenos Aires on 5 August.
The 29-year-old centre missed out on squad selection because of the injury having appeared in the first four matches of the season as he recovered from a knee injury suffered in 2022.
He is the second injury replacement to the 33-man squad following the call-up of flyhalf Handre Pollard for hooker Malcolm Marx. Pollard made a successful return to the team against Tonga.