Will Homer will replace Stuart Townsend in England's U20 World Championship squad after the scrum-half sustained a knee injury over the weekend.
The Exeter Chiefs number nine scored twice against Japan and added his third of the group campaign in the 30-16 victory over Wales on Saturday.
"It is hugely disappointing for Stuart, who has worked extremely hard over the last 10 months to get himself into fantastic shape. He has proved his fitness and his speed of delivery from the breakdown has been exceptional. He was playing very well and I feel for him," said head coach Jon Callard.
Homer, who trained with the squad during the Six Nations and featured in England's hard-fought win against Ireland, arrives in Italy later on Monday.
"We plan for eventualities like this," added Callard.
"Will is experienced, familiar with the squad, and has been training hard with Bath right up until the Aviva Premiership Final so we know he is well prepared and ready to take this opportunity.
"In addition, James Mitchell has had game time already in both pool games and played well during the Six Nations. He brings a lot of energy and tempo so, along with the rest of the management, I am confident that this change will be seamless for the squad."