Canadian captain Sinclair took her tally for her country to 139 after Melissa Tancredi had opened the scoring by netting for the second successive match. Sinclair is now only 19 short of the highest-ever tally in women's football of 158 by Mia Hamm of USA.
Herdman said: "That is the player Christine is. You can contain her for a period of time but when she is on song, she is on song. That is truly the mark of a great player in that some days they are unstoppable. It is good Christine is roaring through the tournament now. That will give her the confidence to keep moving on.
"But Melissa Tancredi scoring goals around her has taken some of the weight off Christine's shoulders. She has scored in successive games now and you are starting to see it is a little bit more than just Christine. We have relied on her for too long."
Canada are now lying in third spot, one point behind Japan and Sweden, who they meet at St James' Park on Tuesday. A draw could take Canada through as one of the best third-placed sides. But Herdman said: "The word 'could' or 'might' is the biggest curse in football and our approach will be to try and win the game."
South Africa are now almost certain to be eliminated after losing their opening two games. Coach Mkhonzana Mkhonza admitted: "This stage is too big for us but it is a good experience and next time we take part, we will be better for the experience.
"We have played well in patches but have not been consistent enough. When we make mistakes, we get punished and, when opponents make mistakes, we do not punish them."