Salah, 34, was withdrawn after 56 minutes in a result that was enough to seal Egypt's place in the last 32. The forward returned to the bench after being replaced, but the sight of the strapping prompted immediate concern among Egypt fans and staff.
Egypt face Group D runners-up Australia in Dallas on July 4, but Salah could miss the match entirely.
Egypt head coach Hossam Hassan was open about the uncertainty surrounding the diagnosis.
"Salah asked to be replaced, but we still don't know the diagnosis," Hassan said.
"Since he felt something, it means there is something. He will undergo scans and we hope for the best."
The lack of a confirmed diagnosis leaves the extent of the injury unclear for now, with scans required before the full picture emerges. If Salah is ruled out and Egypt exit against Australia, his World Cup would be over.
Egypt secured their last-32 place despite only drawing with Iran, a result that ensures their knockout campaign continues — though without any certainty that their most important player will be part of it.
Salah has been one of the tournament's most watched figures. His involvement in Egypt's campaign has generated enormous interest, and the prospect of him missing the Australia fixture will concern the squad and the country's football federation in equal measure.
The scan results will be the defining factor, and Egypt will hope for a positive outcome before their preparations for the Dallas fixture begin in earnest.
Egypt will need him.