Etzebeth left the field in the 26th minute of the 18-3 victory over Scotland in Marseille on Sunday in both teams' Pool B opener.
"We're waiting for the scan," said Erasmus. "I'm sure we'll have 100 percent clarity later this evening.
"It will still be a blow not to have him train for seven to 10 days."
South Africa continue the defence of their World Cup title against Romania in Bordeaux on Sunday, but Erasmus said Etzebeth "would always not be picked" for that game anyway.
"But we would have expected him to have trained fully."
Etzebeth started at lock alongside Franco Mostert against the Scots and was replaced by RG Snyman.
Jean Kleyn and Marvin Orie are the other second-row forwards in South Africa's squad.