Ntini, 28, sustained a torn medial ligament during the third day of the second test on Wednesday. He did not bowl during Australia's second innings on Thursday.
"It was a strange injury," said coach Mickey Arthur. "When he came off in the first innings he noticed a slight swelling just under his knee. "The funny thing about is that he had no pain but there was a little swelling so we thought we'd have scanned and it came back with something to do with the ligament pulling away from the hamstring.