Henshaw, who will need surgery, picked up the injury in the closing stages of the Test at Ellis Park, where South Africa came from 16 points down to beat Ireland 32-26 to level the three-match series 1-1.
Ireland coach Joe Schmidt says he is unlikely to bring in a replacement as he has options within the squad to deal with the loss of the 23-year-old.
Schmidt says: “There are guys on standby so we can go back to them.
“To be honest, if we can cover it internally we probably will at this stage.
“It’s pretty difficult to get them up to speed and to get them down here is a bit of a mission even.”
Meanwhile, Ireland captain Rory Best feels they can still clinch a historic series triumph in South Africa with the right mental attitude in the third and final Test.
He says: “Obviously it [the second Test defeat] was incredibly disappointing and frustrating but we know now it’s 1-1 and we still have a chance to win a Test series over here.
“I think probably the most important thing over the next days is to take this loss on the chin.
“Sometimes you need to just let these losses dwell in you a little bit and then come out the other side stronger.”