What is the remedy for rectal injury due to very hard and large stool?

Fiber, hot baths. "rectal injury" from a large hard stool is most often an anal fissure that will heal in a few weeks with soft stools. Hot baths will help the muscles relax and reduce discomfort. Increasing fiber and water intake to keep the stool soft is important. If it is not getting better within a couple of weeks you should see your doctor.