South Africa captain Siya Kolisi was forced off after just 22 minutes of his comeback game in Racing 92's Champions Cup round of 16 match in Toulouse on Sunday.
The 32-year-old two-time World Cup winner, who was replaced at flanker by Ibrahim Diallo, appeared to have injured his right ankle.
Kolisi, who joined Racing after last year's World Cup triumph, was making his return to competitive rugby after more than a month out following surgery on a finger in his right hand.
Toulouse welcomed back fly-half Romain Ntamack after an eight-month lay-off which ruled him out of France's bid to win the World Cup on home soil.