Samoa's attempts to call up Sosene Anesi have failed.
Winger Ron Fanuatanu has already returned home
Anesi, who has played Sevens for Samoa, declined their advances having pledged himself to New Zealand.
"I talked to him and he said he couldn't make it as he's already signed a contract with the New Zealand RU," Samoan manager Kenape Tuuau said.
"We don't want to jeopardise his chances of an education, because I think that is part of his contract."
Samoa have been hit by injuries to backs Ron Fanuatanu and Terry Fanolua and Anesi had been identified as the ideal replacement.
The 22-year-old winger, who was born in Samoa, played Sevens for his country of birth on the world circuit three years ago.
However, he has pledged himself to New Zealand having made his Waikato debut earlier this year.
In Waikato's recent NPC campaign Anesi was an integral part of the squad and scored a try in the semi-final defeat to Wellington.
"We have wished him well and we are moving on from there," Tuuau added.
"Unfortunately he couldn't make it because he felt he was tied up with his contract not only with Waikato but with the NZRU.
"We have got a very limited number of players and unless you change the rules we will continue to struggle even to find replacements for our injured players."
Fanuatanu injured his cruciate ligaments in the victory over Georgia more than a week ago and Fanolua limped out of the defeat to England.
Coach John Boe, former head of Waikato at Super 12 level, has been frustrated by their inability to call up Anesi.
"We are still trying to get a player from New Zealand, but at the moment we have had no luck. The hurdles are still the same old hurdles," he admitted wearily.
Samoa have been deprived of a host of stars for their World Cup campaign, including a number who are contracted to NPC clubs in New Zealand such as Bradley Mika and Loki Crichton, the leading scorer in the NPC season.
Players based in England refused to join up with the squad for financial reasons.
Despite being hamstrung and down to a squad of 29, Boe is still basking in the glory of his squad's efforts, most notably in their last outing, a 35-22 defeat to England.
"To get so close to the world number one team is just a fantastic achievement for all the Samoans all over the world, but in particular those on the island," he said.
Now all eyes are on the crucial match against South Africa on Saturday, with the winners qualifying for the last eight.
"We have had only a few days to prepare for it whereas the South Africans have had a couple of days and an easier game than us," Boe added.
"We are a bit tired and a bit bashed but nevertheless we will be focusing on the game and doing our best."
Boe's assistant, former All Black legend Michael Jones, described the Springbok match as being the team's "game of their lives".
"It represents a great challenge. We have given the boys two days rest because there is no point flogging a dead horse," Jones said.
"I think the result will boil down to who makes the least mistakes and who retains their discipline the best."