Isshun de Chiryou Shiteita noni Yakudatazu to Tsuihou Sareta Tensai Chiyushi, Yami Healer to shite Tanoshiku Ikiru

From J-Novel Club:
Born a poor man in the slums, Zenos never had opportunities in life, so he decides to forge his own opportunities by teaching himself the curative arts after a chance encounter with a healer. When an adventuring party led by a man named Aston extends him an invitation, Zenos is elated—only to then be treated like dirt by his teammates. Despite Zenos’s diligent efforts to hone his skills to nothing short of perfection, the party deems him useless and kicks him out. Bereft of money and options, Zenos starts an illegal clinic and becomes a shadow healer. Word of his healing genius spreads quickly throughout the city’s underbelly, and as his reputation grows among the dregs of society, knowledge of Zenos's esteem reaches the wrong ears.