Willem Alberts has been ruled out of the Springboks' Rugby Championship tour to Australia and New Zealand and is replaced by Warren Whiteley.
Alberts missed both of the recent Test victories against Argentina because of a hamstring injury and he underwent a fitness test on Thursday afternoon. Although his hamstring is recovering well, he experienced some nerve irritation in his lower back and was sent for scans.
"Willem requires further investigation of the symptoms and will see a number of specialists to determine the optimal management plan to get him back performing on the field as soon as possible," said Bok team doctor Craig Roberts.
Whiteley, who toured with the Springboks to Argentina last week, will join the team in Johannesburg on Friday morning.
Springbok coach Heyneke Meyer said that although Juan Smith was on standby, the player requested more time to work on his match fitness.
"Juan has very high standards and he wasn't happy with his performance in Salta," he said.
"Warren really impressed me last week, he's been playing very well and it made sense to get him back in. Like Willem, he is very effective with ball in hand and also a great option at the back of the lineout.
"More importantly, Warren is a natural No 8, which is a position where we need cover, but he can play flank as well."
The Springboks play Australia in Perth next Saturday and take on New Zealand in Wellington the weekend thereafter.
The squad will prepare in Johannesburg before their departure to Western Australia on Saturday.