The Wallabies are following World Rugby's concussion protocols to the letter.
Toomua's recent concussion against the All Blacks in Sydney was his fourth in The Rugby Championship in three seasons.
He was spelled from last weekend's 29-9 lesson on precision from the All Blacks in Wellington and will undergo cognitive testing after his training return during time-off.
In 2014, the abrasive back was knocked out in back-to-back Tests against South Africa and Argentina and ordered to take more than a month off.
No Toomua for an indefinite period reinforces coach Michael Cheika's backing of Quade Cooper and Bernard Foley as his midfield beyond the Test against the Springboks at Suncorp Stadium on Saturday week.
Combrinck, one of the few success stories in a struggling Springboks side this season, was injured in the first half last weekend and X-rays revealed that he had a fractured fibula.
A South African Rugby spokesman said the back could be out of action until mid-October. The last Championship fixtures are on October 8.
Combrinck, who can also play fullback, debuted off the bench for the Springboks during a June home series against Ireland and quickly became a first choice.
He was a key figure in the Golden Lions team that reached the Super Rugby final for the first time, impressing with his powerful running, strong defending and ability to convert long-range goal kicks.
There are also concerns about the fitness of fly-half Elton Jantjies, who was due to have a groin injury scanned upon the team's return to South Africa.
Coach Allister Coetzee said the Springboks lost because the Pumas made better use of scoring chances.
"They used their opportunities better than we did," admitted the 53-year-old, who succeeded Heyneke Meyer this year as coach of the twice former world champions.
"We had enough chances to create scoreboard pressure, but did not take them. At Test level, you take your chances or you get punished.
"Some penalties were missed and we did not score tries when in a position to do so.
"This defeat hurts. We must regroup and work hard to improve as a team to be ready for Australia in Brisbane."
South Africa face the struggling Wallabies on September 10 and world champions New Zealand a week later in Christchurch.
New Zealand top the table after two rounds with a maximum 10 points, South Africa and Argentina have five each and Australia none.
