The Toulouse winger suffered an ankle injury against Canada, that he then aggravated in the quarter-final win over Japan.
And in the end, he has not come through in time to feature in the clash with Wales in Yokohama on Sunday.
Despite that, South Africa director of rugby Rassie Erasmus backed Nkosi to step up in the absence of his fellow winger.
“It’s disappointing not to have Cheslin available as he has been brilliant for us since we first called him up last year,” he said.
“But we really rate Sbu and he will slot straight in. I am as excited to see what he can do as I would be if ‘Chessie’ were playing. Sbu has been very close to selection as it is.”
That is the only change to the 23 made by Erasmus, with South Africa sticking with the same players who overwhelmed the hosts in Tokyo last Sunday.
Siya Kolisi will captain the team, starting alongside Pieter-Steph du Toit and Duane Vermeulen in the back row, while Faf de Klerk and Handre Pollard continue in the half-backs.
Erasmus has also stuck with a 6-2 bench split for the game with the South African forwards having made a big impact off the bench against the Brave Blossoms.
South Africa team to face Wales:
15. Willie le Roux (Toyota Verblitz)
14. Sbu Nkosi (Cell C Sharks)
13. Lukhanyo Am (Cell C Sharks)
12. Damian de Allende (DHL Stormers)
11. Makazole Mapimpi (Cell C Sharks)
10. Handré Pollard (Vodacom Bulls)
9. Faf de Klerk (Sale Sharks)
1. Tendai Mtawarira (Cell C Sharks)
2. Bongi Mbonambi (DHL Stormers)
3. Frans Malherbe (DHL Stormers)
4. Eben Etzebeth (DHL Stormers)
5. Lood de Jager (Vodacom Bulls)
6. Siya Kolisi (captain, DHL Stormers)
7. Pieter-Steph du Toit (DHL Stormers)
8. Duane Vermeulen (Vodacom Bulls)
16. Malcolm Marx (Emirates Lions)
17. Steven Kitshoff (DHL Stormers)
18. Vincent Koch (Saracens)
19. RG Snyman (Vodacom Bulls)
20. Franco Mostert (Gloucester)
21. Francois Louw (Bath)
22. Herschel Jantjies (DHL Stormers)
23. Frans Steyn (Montpellier)