The absence of three injured first-choice players will be a massive challenge for South Africa in their Rugby World Cup defence, according to three former Springbok captains including Jean de Villiers.

Fly-half Handre Pollard, centre Lukhanyo Am and lock Lood de Jager were all missing from the squad announced in Johannesburg on Tuesday.

However, all three are on standby in case of injuries during the tournament, which starts in France on September 8, with the Springboks facing Scotland two days later.

"The squad is very different to what we would have chosen prior to what we know now," former centre Jean de Villiers told SuperSport.

"If you had to play a World Cup final today you would have selected all three," he said.

John Smit, who captained the Springboks to victory in the 2007 World Cup in France, said the loss of Pollard was a heavy blow as it left the team without an established, match-winning goal-kicker.

"World Cups are won by goal-kickers," said Smit. "We have to figure out who our top three goal-kickers are."

Former star lock Victor Matfield, the most capped Springbok with 127 Test appearances, said the selection of 2019 World Cup-winning captain Siya Kolisi was "fantastic for the country".

But all three former captains shared concern about his match fitness after a long lay-off since knee surgery in April.

De Villiers said defining leadership roles would be important.

"Siya might be the captain of the squad, but he might not be the captain on the field.

- 'Leadership roles' -

"You need to be clear about the leadership roles. We have also lost two leaders in the backline in Pollard and Am."

Matfield added, "Those three (injured players) are a big loss. We only have one recognised fly-half (Manie Libbok), although Damian (Willemse) can play there as well.

"De Jager is the best number five lock in the world. He is the guy running the lineouts."

Smit said Springbok coach Jacques Nienaber and director of rugby Rassie Erasmus had to make tough choices.

"It's difficult to take a guy who is injured to a World Cup because you cannot really send him back. What Jacques and Rassie have done is smart.

"They have picked guys who are fit and able, and they have put the rest on standby. If anything happens to one of the (selected) 33, they can call them up."

De Villiers said South Africa's depth of playing strength "could also be our biggest weakness. I am not sure we know who are best team is."

Matfield said warm-up matches against Wales in Cardiff on August 19 and New Zealand in London on August 25 were important in the build-up to their World Cup opener against Scotland.

"We don't have a pool set-up where we can play guys who are out of form, there are no really easy games. 

"I think in the next two weeks we will see as close as possible to the team that will run on in our first World Cup game."

The Scotland match will be followed by pool games against Romania, Ireland and Tonga.

Group winners and runners-up qualify for the knockout stage. 

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