Steyn had complained of tightness during South Africa's warmup defeat to Pakistan at the Oval in London on Monday, leaving the field to receive treatment after bowling five overs and them going to hospital to undergo a scan.
"At this stage we are unable to determine whether he will be available for the first match against India in Cardiff, however, we will be assessing and treating intensively on a daily basis," said South Africa's manager Mohammed Moosajee in a statement on Tuesday.
There are also doubts over left-arm spinner Robin Peterson, who has swelling after being hit on the left foot in Monday's match.
Quinton de Kock has joined the squad as cover for captain and wicketkeeper AB de Villiers. De Kock is not an official member of South Africa's Champions Trophy squad but will travel with the squad for the tournament.
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