Paul Willemse, 29, was injured in Saturday’s Top 14 loss to Lyon and will miss for the rest of the league leaders’ domestic campaign which includes this weekend’s European Champions Cup quarter-final at La Rochelle.
The second and last Test is on July 9 in Tokyo.
“He is suffering from a tear of the medial lateral ligament in the knee,” Montpellier said.
“We estimate his unavailability to be eight weeks.
“He will finish the season with the medical service and will stay with the squad to support his team-mates on the home stretch,” they added.
Montpellier also without Cobus Reinach
The French Rugby Federation have yet to announce if those involved in the Top 14 final will also to head to Japan.
Last season, players from the league’s semi-finalists were unable to participate in Les Bleus’ three Tests in Australia due to Covid-19 quarantine rules.