Three weeks out from their opening Rugby World Cup game against the All Blacks in Paris, it is the worst of blows and was compounded later the same day when his Toulouse clubmate flanker Cyril Baille was out for several weeks with a calf muscle injury.
France's team is to be announced on Wednesday.
Ntamack's father and former international Emile Ntamack told Midi Olympique they were still digesting the news.
"When you fight for four years for a deadline, you come close and it slips away from you, it's sad. He is combative, I am not worried about his return. But a World Cup in your country is very rare. He will fight to try to make others, but they will not be in France. We're not going to feel sorry for ourselves."
Scotland coach and former international Gregor Townsend, whose side won one and lost one in warm-up games with France, believes the Scots have shown they can handle South Africa's forwards when they meet in Pool play on September 10.
Townsend said, "We're fit enough, we've got the game, and we've got the players to take on any team we come up against, and this [the weekend game against France] was a preparation for what is likely to happen in the World Cup: noisy atmospheres, powerful forwards, especially when we play South Africa and hot conditions.
"Our players thrived in that environment. There were elements where we can improve, and that will come, and it has come because we've seen improvement right throughout the season. Our belief has been shaken at times in games, but against France, the belief was there. Whether it was individuals or collectively, we knew we were coming back.
"It is a big disappointment that we didn't get anything out of that game whether it is a draw or a win. We know that if that was a World Cup match we'd be sitting here absolutely gutted, but it is a build-up to the World Cup so there are lots of positives out of that performance."
Uruguay coach Esteban Meneses, whose side will play the All Blacks on October 5, has hit out at World Rugby's eligibility laws before the World Cup.
Meneses said decisions that allowed the likes of Samoa and Tonga to select players from New Zealand and Australia was a continuation of policies that prevented sides like Georgia being prevented from playing in the Six Nations Under-20 tournament despite being ranked ahead of Scotland and Italy.
Meneses said Test teams should be representative selections of a country, and the eligibility laws threatened the identity of countries.
If Tonga had problems being competitive for World Cups, World Rugby should help it develop better players rather than taking a backward step by allowing outside players to be selected.
He said, "Why aren't Uruguay or Georgia rewarded if they do things well?"
Uruguay or Georgia were punished for being productive, while Tonga and Samoa could choose Test players from rival countries and be rewarded.
