The former British and Irish Lion, 30, is battling a calf injury which ruled him out of the latter part of last season, and he was not named in coach Warren Gatland’s squad for the side’s warm-up clash with Argentina on Saturday.
“Unfortunately, Gethin's calf is still not 100%," Gatland revealed. “We have had him rescanned, and there is some damage in the calf.
“I would say it probably rules him out of contention for the South Africa game if selected.
“He is as frustrated as anyone. He has worked really hard, he just hasn't been able to get himself right.”
Gatland will name his 30-man World Cup squad on August 22.