Evans, who has been capped 33 times by Wales, has been out of Warren Gatland's plans since his move from Ospreys and a serious injury could harm his World Cup chances even further.
"It's a contact injury and it's a contact game - unfortunately you're going to pick up those injuries," said Bristol Director of Rugby Andy Robinson.
"Ian remains a key member of the squad and he has a part to play as we approach the play-offs. His experience and input will be invaluable to the team on the training ground."
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