What is the importance of the arm sling the narrator wears after he is attacked in the novel, All Our Names?

After the narrator leaves the hospital from being attacked, Isaac takes him to the home of Joseph Mabira. The narrator is wearing an arm sling to protect his damaged arm and his broken ribs. On the third day of occupying the house, Isaac gets the home ready for Joseph's return with the desire that everything be absolutely perfect. When he sees the narrator in the arm sling, Isaac asks if it is absolutely necessary to wear it, as he perceives it to be a sign of weakness. Out of wanting to ensure that Isaac makes a good impression on Joseph, the narrator removes the sling, no matter how damaged and hurt he still is, causing Isaac to be even more proud of his friend.