South Africa coach Jacques Nienaber said scrum-half Grant Williams would be out of action "for at least two weeks" after being concussed after just 10 seconds in a Rugby Championship win over Argentina on Saturday.

"Grant was out cold so I think there is definitely concussion. I don't know the extent of it, but he will probably be out for at least two weeks," Nienaber told a press conference.

Much of the build-up to the final-round match, won 22-21 by the Springboks in Johannesburg, had centred on dynamic Williams getting his first start for the Rugby World Cup title-holders.

But just 10 seconds after the kick-off, Williams was concussed after a collision with Pumas fullback Juan Cruz Mallia and stretched off.

He later appeared on a touchline bench with the South African replacements to applause from the Ellis Park stadium crowd.

Assuming South Africa take three scrumhalves to France for the 8 September- 28 October global showpiece, 2019 World Cup winner Faf de Klerk is a certainty.

Williams is competing with his Sharks teammate Jaden Hendrikse, Cobus Reinach and Herschel Jantjies for the other two places.

Hendrikse, Reinach and Jantjies were later included in a 26-man squad for a World Cup warm-up international against Argentina in Buenos Aires on 5 August.

"It was not the nicest game, but we needed to grind it out with a yellow card in the last 10 minutes," said Nienaber, referring to the 73rd-minute sin-binning of centre Damian de Allende.

"Our kicking game was definitely not as good as it could have been. Argentina handled the game between the two 10-metre lines very well. They wait for you to make mistakes."

South Africa spent most of the second half defending after leading 15-9 at half-time and Nienaber hailed the Springboks for preventing the Pumas scoring a try until the 75th minute.

'INCREDIBLY PROUD'

"Being put under pressure like this will probably happen in the World Cup. I was incredibly proud of the outstanding attitude in defence," said the coach.

"I think the World Cup is going to be incredibly close and you can easily become shell-shocked when the opposition has all the momentum."

The Ellis Park test was the last as Springboks coach in South Africa for Nienaber, who is joining Irish outfit Leinster after the World Cup.

Argentina coach Michael Cheika said the Pumas played better in Johannesburg than in Sydney two weeks ago when they snatched a last-gasp 34-31 victory over Australia.

"Our goal is to keep improving. There were some areas that we know are Springbok strengths and we managed to contain those, like the maul," he said.

"We handled that well, we worked really, really hard. I am really pleased with the way we played and stayed in the game.

"Character is one thing this team has in spades. They are good people and friends, a tight-knit bubble. We are changing the dynamic in a positive way and it is a great team to be part of."

Australia-born Cheika was unable to give an immediate update on injured lock Lucas Paulos, who suffered concussion late in the first half.

Three hours before the match in Johannesburg kicked off, New Zealand won the Championship by crushing bottom-placed Australia 38-7 in Melbourne. South Africa finished second.