The influential back rower ruptured her anterior cruciate ligament in her right knee during the second half in Tuesday’s 15-10 defeat to South Africa at the Surrey Sports Park.
An MRI scan confirmed that surgery will be required on the 44-times capped Nicholas who now faces up to nine months on the sidelines in rehabilitation.
“It’s a bitter blow to lose someone of Cat’s experience,” said Wales head coach Jason Lewis. “She is an influential member of the side and a key component of our pack. She’s been playing really well so it’s obviously a big blow lose such a key player.”
Twenty-year-old UWIC and Scarlets No8 Vici Owens has been added to the squad as a replacement for Nicholas.