Botha was a surprise inclusion in the side named on Thursday for the world champion Springboks who have been hit with a rash of injuries ahead of their rugby World Cup Pool D clash.
"I will still feel a bit of stiffness but it's like old farm vehicles in the morning, it smokes a bit when you start it up but when you drive it around for half an hour and it gets hot, it can go," said 31-year-old Botha, who has been suffering a chronic Achilles injury.
"Hopefully it will (be the same for me). I will definitely start warming up before the others do."
Botha laughed when asked about the team's doubtful comments about his recovery all week, which included coach Peter de Villiers suggesting the lock's entire World Cup could be over.
"At this stage I'm very happy. I trained hard last week on the rehab-ing side and the medical (team) did a lot of work on my left Achilles," Botha said.
"I gave it a go on Tuesday morning and it was happy to get through the training session.
"I'm going on to the pitch feeling good ... there is no doubt in my mind (and) when I pull on that green and gold jersey, I give eveything."
Botha's return to the pack will be welcomed by De Villiers, who had to make four forced changes from the team that beat Wales with vice-captain Victor Matfield (hamstring), utility back Butch James (hip flexor), winger Bryan Habana (patella tendon) and centre Jean de Villiers (rib cartilage) all ruled out.
Replacement lock Johan Muller will undergo a late fitness test on a tight hamstring. Francois Louw would replace him in Wellington if he was ruled out, De Villiers said.
"We have a few injuries but I think it's a brilliant exercise for (captain) John (Smit) to blood some of the other players," De Villiers said.
De Villiers said that the versatile Danie Rossouw, who had played lock and loose forward already this year for the Springboks, would have the added responsibility of calling the lineouts on Saturday.
Full-back Frans Steyn has moved to inside centre to cover for Jean de Villiers' absence, with talented youngster Pat Lambie coming into the side as the last line of defence while Odwa Ndungane replaces Habana on the left wing.
Botha replaces Matfield in the second row, while Gurthro Steenkamp and Tendai Mtawarira swap roles with Steenkamp starting at loosehead prop while Mtawarira shifts to the bench.
The Springboks beat Wales 17-16 in their opening match while Fiji beat Namibia 49-25. Samoa are the other team in the group nicknamed the 'Pool of Death'.
South Africa: 15-Pat Lambie, 14-Odwa Ndungane, 13-Jaque Fourie, 12-Frans Steyn, 11-JP Pietersen, 10-Morne Steyn, 9-Fourie Du Preez; 8-Pierre Spies, 7-Schalk Burger, 6-Heinrich Brussow, 5-Danie Rossouw 4-Bakkies Botha, 3-Jannie Du Plessis, 2-John Smit (captain), 1-Gurthro Steenkamp
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