On August 7, 2018, Hernández allowed 11 runs in six innings, in a loss to the Texas Rangers. This raised his ERA to a career-worst 5.73 in 23 starts. After the game, manager Scott Servais would not commit to his next start in the rotation. On August 9, Servais removed Hernández from the starting lineup. On August 10, Hernández was removed from the starting rotation and transitioned to the bullpen. He made his first appearance as a reliever on August 14 against the A's when starting pitcher James Paxton was struck by a line drive. Hernández temporarily returned to the rotation with Paxton on the disabled list. Hernández finished the worst season of his career, going 8–14 with an ERA of 5.55 in innings.