The 27-year-old defender was forced off just before half-time during Saturday's goalless draw with Manchester City after dropping to the floor and clutching his right leg.
It is hoped the injury is a muscle problem, likely hamstring, but Stephens has struggled with a reoccurring knee issue recently, giving his manager Hasenhuttl some cause for concern.
“I don't know what it is but he has had knee problems. Hopefully it is not too bad," said Hasenhuttl.
“He was in perfect shape, he was a really important player for us. So early in the season an injury would not be good for us."
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