Zenmetsu End wo Shinimonogurui de Kaihi shita. Party ga Yanda.

Walker, a man reincarnated as a teenage adventurer, is suddenly attacked in a dungeon by a high-level monster and realizes he's in the dark fantasy world of a manga he read in his previous life. Not only that, but he isn't the protagonist... He's an ill-fated side character, meant to die along with his party to establish the setting's grim tone! Walker fights tooth and nail for his friends, and although he loses a leg and an eye in the process, he manages to save everyone from almost certain doom. But as he adjusts to a new life with permanent injuries, his female comrades, shaken by his near-death experience, begin to act strangely and overprotectively toward him!

(Source: Yen Press)