Schreuder is the second no.9 replacement to enter the Bok camp this week with the Cheetahs Piet van Zyl replacing injured Bull Jano Vermaak.
Hougaard will undergo scans for a knee injury this week but is not expected to need surgery. He will be withdrawn from the squad as a precautionary measure.
“At this stage Francois doesn’t require surgery but will be re-evaluated in two weeks’ time to see if the injury has settled sufficiently for him to resume playing,” said Springbok team doctor Craig Roberts.
Springbok coach Heyneke Meyer said: “Losing two No.9s in such a short space of time is obviously a setback, but I’ve always rated Piet van Zyl and Louis Schreuder as two of the best young scrumhalves in South Africa.
“Piet’s attitude has been fantastic since he joined us on Sunday and he’s worked hard on the training field and off it, making sure he gets to know and understand our plays as quickly as possible. He’s been playing great rugby for the Cheetahs and I have a lot of confidence in him.
“Although Louis has not officially been part of our set-up yet, he has helped out at two of our training camps in Cape Town before, so coming into the squad will not be all new to him,” said Meyer.
In other news, Francois Louw has been released from the squad in preparation for his wedding day.