The Leeds Tykes player was himself a late call-up after England's first-choice No. 2 Steve Thompson withdrew with an injury earlier this week.
Regan will be replaced by Andy Titterrell of Sale Sharks, who comes off the bench.
Andy Long of the Rotherham Titans now comes into the squad, replacing Titterrell in the replacements.
Regan joins Phil Vickery, Charlie Hodgson, Lewis Moody, Iain Balshaw and Mike Worsley in the sick bay, all of whom were forced to withdraw from the squad through injury.
