Mitchell replaces injured Van Poortvliet in England's World Cup squad
Van Poorvliet had established himself as the probable first-choice scrumhalf for the tournament, with experienced due Ben Youngs and Danny Care also in the squad, but needs surgery on the injury.
“It’s really bad luck for Jack, and we send him our very best wishes for a swift and successful recovery,” said England coach Steve Borthwick.
Owen Farrell will find out the length of his ban on Tuesday following his red card for a dangerous tackle in the Wales match.
He is likely to miss at least four games, ruling him out of England's remaining two warm-ups against Ireland and Fiji and their first two World Cup games against Argentina and Japan - their biggest pool rivals.