The button should appear next to the pop up window selection and setting button. I wouldn't be surprised if there's no button for half injured players. I think there is because I once spotted a "make sub" just before it vanished. But I've never used that, I keep an eye in the squad ratings window for the orange icon, because if I rely on seeing the line I'll have players on the pitch until their leg detach from their bodies like that Simpsons episode.