Uncapped off-spinner James Tredwell was called into the England squad as cover on Thursday after Graeme Swann and Stuart Broad were ruled out of the first two One-Day Internationals through injury.
Spinner Swann has a side strain and pace bowler Broad a shoulder injury.
Tredwell, 27, will vie with leg-spinner Adil Rashid, who conceded 25 runs in his only over in the second Twenty20 international, for the opening one-dayer on Friday.
"James Tredwell is definitely an option. Being a right-arm off-spinner and with quite a few left-handers in the South African line up, his ability to spin the ball away is important," England captain Andrew Strauss told a news conference at the Wanderers.
"He has deserved his chance, he has performed very well for Kent over a long period of time and is a very canny one-day operator.
"But Adil Rashid is improving all the time and the more exposure he gets the better. He needs to play too, but it's important that we get the right player for the game. Clearly, it's important that we have both options." Strauss said.
South African coach Mickey Arthur said he expected 21-year-old Rashid to play.
"He's the logical choice, but it will be interesting to see if Tredwell steps ahead of him. It would justify my saying that it doesn't look like England have faith in him (Rashid)," Arthur said.
England have won their last five ODIs against South Africa, including a shock triumph at Centurion in September which knocked the favourites out of the Champions Trophy.
"The 4-0 series win in England was a long time ago, in very different conditions. But we will take some confidence from beating them in a pressure situation out here in the Champions Trophy," Strauss said.
"South Africa at home are hard to beat, they've shown that time and time again, but I like the way we've prepared and I like the brand of cricket we've played.
"I'm very happy with where the squad are and I don't think I've ever seen an England squad more motivated to do well," he added.
England won the first, rain-affected Twenty20 international by a single run last week. South Africa reversed some of that momentum by scoring 241 on their way to an emphatic 84-run triumph at Centurion in the second match.
"We will take some confidence from scoring at 12 an over against them and the England bowlers would have had to go back, think about and analyse their performance," Arthur said. "I know we would have after a day like that.
"We played particularly well that day, but England have played very well against us before that. They are still a work in progress, but they are a very good side and they can beat anyone on their day," he acknowledged.