Shaw is on his way to Brisbane after Grewcock, who missed the first three World Cup games with a broken toe, sustained a hand injury in the 111-13 win over 'Los Teros'.
The Bath forward will undergo an X-ray on Monday before any decision about replacing him is made.
"Simon Shaw has been called up as cover for Danny Grewcock, who injured his left hand during tonight's game," said an England spokesman.
"Further X-rays will be conducted tomorrow [Monday] and subject to the result of these a decision is anticipated on whether to approach the Rugby World Cup to consider Danny Grewcock's replacement."
England coach Clive Woodward thanked the London Wasps management for allowing Shaw to travel and miss the Zurich Premiership game against Newcastle Falcons on Sunday.
"I'd like to thank Warren Gatland and London Wasps for their full support over this situation."
Shaw, who was unlucky to miss out on the original World Cup squad, is the second player to be flown out as cover during the tournament by England.
Bath's Martyn Wood spent three days in Perth during October – when Woodward had a potential scrum-half injury-crisis – only to return home when Kyran Bracken and Matt Dawson returned to full fitness.