Tanvir had been set to join up with the Royals after playing for Pakistan in their recent Twenty20 series in Sri Lanka for tonight's opening game with Warwickshire at Edgbaston.
But he has been added to the Pakistan squad for their ongoing one-day international series against the Sri Lankans after an injury to Nasir Jamshed.
Worcestershire chief executive David Leatherdale confirmed: "Sohail has been added to Pakistan's one-day squad and will remain in Sri Lanka until Monday.
"We are trying to organise a visa so he can fly direct to England and avoid having to go back to Pakistan first and hope he will only miss the first two Twenty20 games."
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