Steyn is recovering from a heel injury and told the South African media that he would not be fit to face the tourists in Cape Town from next Thursday in a fiery series deadlocked at 1-1.
"I cannot imagine another Test where I need to retire halfway through it because my foot is bruised," said 34-year-old Steyn.
"I cannot let my team-mates down, or my country. It has happened a couple of times now," said the injury-prone pace ace who needs three Test wickets for a South African record. "As a fast bowler, I need to land on my front foot. Putting eight or nine times my body weight on that foot... it can only handle so much."
Steyn said he hoped to be available for the potential series-deciding final Test at the Wanderers in Johannesburg from March 30.
The veteran offered sympathy to Rabada, who will miss the final two Tests for brushing the shoulder of Australia skipper Steve Smith unless he wins an appeal to be heard Monday.
"Kagiso is 22 years old. We all make stupid mistakes at that age. I did stupid things when I was 22. We all make these mistakes,” he said.
Rabada is the fifth player to be punished in a fiery series after Australians David Warner, Nathan Lyon and Mitchell Marsh and South African Quinton de Kock.
Australia won the first Test in Durban by 118 runs and South Africa triumphed by six wickets in Port Elizabeth to level the series.
Long-serving Morkel says he will retire from international cricket after the tour by Australia, which is confined to Tests.
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