The Dragons number eight has been suffering with back spasms and has not participated in squad training sessions in Cape Town.
Better news for Ruddock is that Hal Luscombe's damaged thumb has been put in a splint, but he and fellow thumb injury victim Sonny Parker are expected to be fit to play in Pretoria. Dwayne Peel's ankle injury has responded well to intensive treatment, and he is hopeful of being involved.
Eight of the squad arrived in South Africa before Wales managed to level the two-match series with Argentina last Saturday, and Ruddock said he was encouraged by the 'steel' shown by the players in Buenos Aires, but he hopes those who were released early from the squad will show a greater hunger to be involved against the Springboks.
'I'm sure we will see a lot of enthusiastic guys who have been left out of a couple of Tests who will be chomping at the bit to be in the Test on Saturday. That will put a bit of pressure on the boys who've just got here to up their standards to make sure they don't miss out on selection. I think it'll be positive all round.'
'My message to the players is 'I'm not here to make you happy but to see if you work well and whether mentally you can handle disappointment and pressure','
'If they don't get their chance this time, I want them to be strong candidates for selection for the November Tests, for the Six Nations and possibly the World Cup.'
Wales have never beaten the Springboks on South African soil. They lost 19-8 in their most recent match against the 1995 World Cup winners.