Mapimpi was ruled out of the remainder of the tournament with a facial fracture sustained in the 49-18 victory over Tonga on Sunday that took the Springboks to the brink of a quarter-final place.
Am is a key creative player for the side but was suffering with a knee injury when the squad was announced on August 8th.
Head coach Jacques Nienaber said at the time Am could be called on at a later stage should they need a replacement.
He is the second medical switch for the Boks after hooker Malcolm Marx was ruled out with a knee problem, allowing Handre Pollard to return to the squad. He played his first minutes against Tonga.
Jesse Kriel was the only specialist outside centre in the squad prior to Am’s return, though young Canan Moodie has been covering the number 13 and 14 jerseys at the tournament.
South Africa will earn a quarter-final place unless Scotland beat Ireland by 21 points or more and the latter get a bonus point for four tries when the teams meet in Paris on Saturday.
The Springboks have completed their pool matches and should they reach the knockout stages will next be in action on either October 14th or 15th.
