Widnes's Lee Paterson will miss the rest of Scotland's World Cup campaign after suffering an anterior cruciate knee ligament injury against France.
"It is a devastating blow. Lee has been an integral part of our squad," said Scotland coach Steve McCormack.
"A scan showed damage and the best course of action for him is to return home for further investigation."
Scotland must beat Fiji by a wide margin in their final pool game to have any chance of reaching the semi-finals.
McCormack's side began with a 36-18 loss to the French.