In Romance, when Xiahou Dun was shot in the eye, he proclaims:

"父精母血，不可弃也"

"Essence and blood of my father and mother, must not be abandoned

It's essentially a demonstration of Confucian virtue and filial piety, with the traditional concept of the body being sacred, being produced from one's parents and ancestors, who deserve utmost respect and proper treatment. Confucianism expounds the idea of "humaneness", to be virtuous or to "be a good human", and the importance of proper behavior and conduct in relationships. It starts in the basic foundation of society, the household, and by maintaining proper relations with your parents, you can extend that to your larger family, your community, your nation. A father should act as a father to their son, a son should act as a son to their father. Being a good father and being a good son extends to being a good teacher to a student, student to teacher, citizen to Emperor, Emperor to citizen, etc. That allows for the functioning of an ideal society.

In the Classic of Filial Piety, there's a passage explaining the importance of the body and its relation to virtue and piety:

子曰：“夫孝，德之本也，教之所由生也。复坐，吾语汝。身体发肤，受之父母，不敢毁伤，孝之始也。立身行道，扬名于后世，以显父母，孝之终也。夫孝，始于事亲，中于事君，终于立身

The Master said, "Now filial piety is the root of (all) virtue, and (the stem) out of which grows (all moral) teaching. Sit down again, and I will explain the subject to you. Our bodies - to every hair and bit of skin - are received by us from our parents, and we must not presume to injure or wound them. This is the beginning of filial piety. When we have established our character by the practice of the (filial) course, so as to make our name famous in future ages and thereby glorify our parents, this is the end of filial piety. It commences with the service of parents; it proceeds to the service of the ruler; it is completed by the establishment of character.

So, when Xiahou Dun was struck by an arrow in Romance, Luo Guanzhong in an effort to romanticize characters and highlight heroic qualities, depicts Xiahou Dun bravely swallowing his own eye so as to "honor" his parents. He recognizes that his eye, as anything on one's body, is an extension of his parents and ancestors, and his duty to revere them. Throwing it away would be akin to throwing away your own parents' bodies, and so he consumes it as a way of maintaining it within himself, and as an extreme form of righteousness and valor.

There are other passages in Romance that depict consuming human flesh as Confucian piety. I wrote a related piece in this thread that you may be interested in for more details, explaining why the loyal subject Liu An killed his own wife to feed her to his ruler Liu Bei.