Castres signed South African back-row forward Wessel Jooste as emergency cover on Tuesday with both regular starters Pedrie Wannenburg and Yannick Caballero out injured.
Jooste, 29, already has Top 14 experience with Bourgoin, where he played for seven seasons, and in England with Leicester Tigers.
Castres called on Jooste -- who trained with the high-flying Top 14 outfit on Tuesday and could feature in Friday's home game against leaders Toulon -- with Wannenburg out for the season and Caballero out of contention with a knee injury.
Their resources were further diminished with South African-born flanker Antonie Claassen called up to France's Six Nations squad for Saturday's game with England.
"I was in contact with Castres on Saturday and I had no hesitation in coming back to France, a country I like a lot," he said.
Castres are presently fourth in the table after 18 matches, 16 points adrift of Toulon.