Zouma, 27, was sent for a scan on Monday, ahead of Thursday's crucial Europa League quarter-final, second leg, against Lyon on Thursday.
West Ham are level in the tie at 1-1 but will already be without left-back Aaron Cresswell following his controversial red card at the London Stadium last Thursday.
“Kurt has twisted his ankle," said manager David Moyes. “We don't know how bad it is.
“We'll get him checked. Coming off is not a good sign but hopefully it's not too bad."
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