England prop Mako Vunipola will be sidelined for 10 days after re-injuring his hamstring during the side's record 57-15 rout of Ireland in a World Cup warm-up match last weekend.
Vunipola, who was injured in Saracens' Champions Cup final win over Leinster in May and required surgery that ruled him out for the rest of the season, made his return at Twickenham on Saturday.
But he lasted only a few minutes as a replacement before he hobbled off.
"Mako Vunipola has a small tear of scar tissue which will require approximately 10 days of rehabilitation," the RFU said in a statement.
"Ben Moon will join up with the squad as a temporary replacement to train in Treviso."
Vunipola has struggled with injuries of late, with an ankle injury ruling him out of the final three Six Nations matches earlier this year.
England play one more warm-up match against Italy on 6 September and the 28-year-old is unlikely to be risked before the squad heads to the Rugby World Cup in Japan. England begin their campaign against Pool C opponents Tonga on 22 September.