Chiyu Mahou no Machigatta Tsukaikata

Ken Usato is accidentally transported to another world alongside two students who were deliberately summoned as heroes. At first, he appears to be little more than an extra caught in the spell, until he is discovered to possess an extremely rare talent for healing magic. That news attracts Rose, the terrifying captain of the kingdom's Rescue Team, who drags him into training unlike anything expected of a healer. By repeatedly healing his own body, Usato is pushed beyond ordinary limits until he develops the strength and stamina to run into battle, reach the wounded, and carry them back alive. An accidental summoning becomes the start of a path unlike that of any conventional hero.