Using the stomach on the venator corpse will yield the message "While the stomach presumably came from this venator, the corpse has been stitched up again, preventing you from putting it back. That said, you're not sure why you'd want to put it back anyway."
After using the sharp knife on the venator corpse, using the stomach yields a different message: "The prognosis for this venator is poor. Putting the stomach back in it is unlikely to change that."