Underhill has been ruled out with a hip injury ahead of the 28-man squad’s assembly at England football’s St George’s Park base tomorrow.
England begin their tournament a week on Saturday against Scotland at Twickenham.
Willis, who made his England debut during the Autumn Nations Cup, steps up from a “shadow squad” operating under the main group.
Marler and Launchbury were replaced by Wasps prop Tom West and Bath lock Charlie Ewels in the main squad.
As a result, three players have been added to the shadow squad: Exeter prop Alec Hepburn and the Northampton duo David Ribbans and Lewis Ludlam.