Toulouse's Ntamack out of crunch Champions Cup match against Sale
The 26-year-old has missed Toulouse's last two matches, including Sunday's defeat at Saracens, because of an injury to his lower back.
Although he trained with his teammates on Tuesday, he limited himself to light exercises that didn't put strain on his back and will be absent on Saturday at Ernest-Wallon against Sale.
He will undergo further tests next Monday, the club confirmed.
Six-time champions Toulouse are currently fourth in Pool 1 and, after losing both of their away games, need to beat Sale to be certain of qualifying for the last 16.
Anything less will allow Durban Sharks, currently two points behind, the possibility of leapfrogging them into the fourth qualifying spot when they host pointless Clermont earlier on Saturday.
In Ntamack's absence either Scotland international Blair Kinghorn, who played against Sarries, or France full-back Thomas Ramos is likely to fill in.
Alternatively, coach Ugo Mola may hand the keys to No. 10 to 20-year-old centre Kalvin Gourgues who filled in against Perpignan after Ntamack withdrew.
Ntamack's extended absence could have repercussions in the France team with national coach Fabien Galthie due to name his opening Six Nations squad early next week.
France begin the defence of their Six Nations crown against Ireland on February 5 at the Stade de France.