The 24-year-old wicketkeeper injured the index finger on his right hand while fielding in South Africa's eight-wicket win in the second test in Wellington and underwent scans earlier this week.
He missed training on Wednesday and Thursday and while scans determined the finger was not broken, he had damaged tendons.
De Kock trained on Friday with strapping and a protective splint and South Africa captain Faf du Plessis said as far as he was concerned the aggressive batsman was too important to leave out for the series-deciding match starting on Saturday.
"It's a big game. Quinton is someone who you don't just replace. From my side, I am trying to have Quinton in my team all the time," du Plessis told reporters in Hamilton.
