He's done a medial ligament
The All Blacks 28-13 win over Argentina on Saturday came at a high price, with the team losing captain Richie McCaw to injury for at least four weeks.
The influential All Blacks skipper had to be assisted from the field around the 60-minute mark and is unlikely to take any further part in the All Blacks defence of the Rugby Championship crown.
Victory celebrations were tempered as the full extent of the injury to McCaw became known.
"He's done a medial ligament and it looks like he could be out for four or five weeks," coach Steve Hansen said after doctors finished their examination of McCaw.
"At this stage the doc thinks it's four or five weeks. Like all injuries he could wake in the morning and feel a lot better but I'm not thinking it's going to be that.
"We might as well get it out now and get on with it."
McCaw's absence means Sam Cane is most likely to start against the Springboks in their fourth-round match in Auckland next weekend.
Hansen could not confirm who would be added to the squad but indicated it would be Canterbury Crusaders flanker Matt Todd.
Todd has been training with the All Blacks and made one brief Test appearance in the final 10 minutes of the third Test against France in June.
The All Blacks made two other forced replacements against Argentina, with prop Tony Woodcock pulled from the field at half-time for illness and his replacement Wyatt Crockett to be assessed for possible concussion received late in the game.
The other injury to the world champion All Blacks was to their pride as they failed to dominate Argentina as expected.
Their preference to play a fast running game lacked fluency in wet conditions and against the determined defence of the Argentinians, leading to multiple handling errors.
Although they were able to command the line-outs, their scrum creaked woefully against the powerful South American pack.
Argentina led 7-0 early when a basic handling error by debutant centre Francis Saili let Juan Manuel Leguizamon in for the easiest of tries.
The game eventually turned in the 22nd minute when Eusebio Guinazu was yellow-carded for a professional foul.
During his 10 minutes in the sin-bin Aaron Smith scored two tries, and with Dan Carter landing a conversion and a penalty the All Blacks were suddenly 15-7 ahead.
Julian Savea scored the All Blacks third try in the second half with the remaining all Blacks points coming from the boots of Carter and Beauden Barrett.
Bordeaux-Begles flyhalf Nicolas Sanchez kicked one conversion and two penalties for the Argentina.