Farmer was stretchered off in the 3-1 loss to the hosts on Saturday (28 November) after a clash of heads with an opponent late in the second half, leading to a mild concussion.
He did not complete the training session on Monday afternoon (30 November) and had to watch from the sidelines.
The medical department is keeping a close eye on him.
“He got a hard knock but suffered a mild concussion, which is good news. He has improved a lot from Saturday but we had to withdraw him from the training session as a precaution and could not risk him in the match with Zambia. Hopefully he will have fully recovered for the next fixture on Saturday against Tunisia,” said team medico Dr Thulani Ngwenya.
The defender is positive about the outcome of the encounter against Zambia.
“To be honest we did not really play bad in the first game, just that we were not composed in the box and our finishing let us down. We need to take our chances better and we will rectify it in the game against Zambia. We must go all out and make sure we do not concede, and score as many goals as possible,” added Farmer.
“Losing is not option at all. We are well prepared and we know what we are up against so have to work on our strengths and let Zambia worry about us.”
The two nations face off on Tuesday, 1 December at the Leopold Senghor Stadium in Dakar, Senegal.
Kickoff is at 17h00 South African time.
Both teams lost their opening matches.
Tunisia and Senegal meet in the other Group A match.