The 24-year-old Ntamack ruptured cruciate ligaments in his left knee in the second half of the 30-27 victory over Scotland in a World Cup warm-up on Sunday, the team said following scans.
Matthieu Jalibert is set to fill in as Ntamack's understudy when the tournament begins in France on Sept. 8.
“Romain, we are thinking of you a lot,” the French rugby federation posted on X, formerly known as Twitter. “The whole group is supporting you. We wish you a good recovery.”
In his own post, Ntamack wrote: “I'll be back.”
