De Klerk and Jantjies didn't play in the two Tests against Argentina played in South Africa due to injuries but have been cleared to return for the Springboks' Tests with Australia and New Zealand.
Another halfback, Grant Williams, has been called into the squad as cover after learning Jaden Hendrikse suffered a broken leg last weekend against the Pumas.
Springboks coach Jacques Nienaber said, "It is sad to lose a player like Jaden who was beginning to find his feet in the system and starting to make his mark in Test rugby, but, unfortunately, injuries are part of the game.
"We are lucky to have a handful of talented scrum-halves in the country, and, as coaches, we have been impressed with Grant's form this season.
"He will certainly benefit from being in the squad and being exposed to our structures and systems, and we are looking forward to what he has to offer," he said.
The Springboks will also have No8 Duane Vermeulen in their squad in Australia.
Nienaber said Vermeulen was close to the final stage of his rehabilitation after ankle surgery.
"Duane has been a handful for us at training, and he is keen to take contact again," he said.