Willis has been ruled out of the remainder of the tournament with a neck issue.
Abrasive Underhill, 27, a starter for England in the 2019 final, comes in as the team prepares for next weekend's last-eight tie with, in all likelihood, Fiji.
"Sam Underhill has been added to England's Rugby World Cup squad in France," the RFU said.
"The Bath Rugby back-row forward replaces Jack Willis and will join up with the squad in Lille today ahead of England's Rugby World Cup quarter-final on Sunday 15 October in Marseille," it added.
Underhill faces competition from the likes of Tom Curry and Courtney Lawes for a space in the starting lineup against Fiji on the Mediterranean coast.
The Pacific Islanders need just a point against Portugal later on Sunday to claim their spot in the knock-outs.