Back-row forward Sergio Parisse, the Stade Francais captain, will be out for "three to five weeks" with a calf injury, the club's sporting director Heyneke Meyer said on Thursday.
For the top-of-the-table game against Clermont the 34-year-old, who has played 134 times for Italy, will be replaced at No.8 by South African Willem Alberts and as captain by Remi Bonfils on Saturday.
Jules Plisson is promoted to vice-captain and, because Bonfils is a hooker and likely to be replaced, would expect to be in charge by the final whistle.
Parisse was hurt during the victory over Bordeaux-Begles last weekend, Meyer said, although the long-time Italy captain finished the game.
"It's a big setback for us because he's played really well and he's a very good captain," Meyer said.
"But this is not an excuse... Remi and Jules are two very good leaders, we need more than one leader and it's a good opportunity for them."
Parisse was only able to play against Bordeaux after the red card he picked up the previous week at Perpignan was overturned.