Southern hemisphere rugby body SANZAAR are probing a challenge by Argentine Juan Cruz Mallia which knocked South African Grant Williams unconscious in a Rugby Championship match.

Full-back Mallia collided with scrum-half Williams just 10 seconds into the final-round fixture in Johannesburg last Saturday after the Springbok fielded the kick-off and cleared the ball.

On Monday, SANZAAR said in a statement it was investigating what it called "a red card offence", meaning Mallia should have been sent off.

 "Juan Cruz Mallia of Argentina has been cited for alleged foul play during a Rugby Championship match at the weekend," the statement said.

"Cruz Mallia is alleged to have contravened Law 9.11: Players must not do anything that is reckless or dangerous to others, during the match between South Africa and Argentina.

"Upon further review of the match footage, the citing commissioner deemed, in his opinion, the incident had met the red-card threshold for foul play.

"The case is to be considered in the first instance by the SANZAAR foul play review committee, which will take place (this) Tuesday via video-conference."

Williams, making his first start for the reigning world champions after three appearances off the bench, was driven from the field on a stretcher.

Medical attention led to a quick recovery and later in the first half the scrum-half joined the South African substitutes on a pitch-side bench.

Speaking after the Springboks edged the Pumas 22-21 to finish runners-up behind New Zealand, head coach Jacques Nienaber said Williams would be out of action "for at least two weeks".

After viewing the incident on a big screen, Irish referee Andrew Brace ruled that "no foul play had occurred".

Argentina and South Africa meet again on Saturday, this time in Buenos Aires, in a warm-up match ahead of the September 8-October 28 Rugby World Cup in France.

South Africa, winners of the World Cup a record-equalling three times, will also play Wales and New Zealand before the global showpiece while Argentina face Spain in another warm-up match.