On July 21, 2015, during a game against the Pittsburgh Pirates, Vargas was forced to leave the game after throwing only 26 pitches, with an apparent injury to his throwing arm. The next day, Vargas was diagnosed with a torn UCL in his left elbow, forcing him to undergo Tommy John surgery and miss the remainder of the 2015 season and much of the 2016 season. For 2015, he was 5–2 with a 3.98 ERA in nine starts. Although his season ended prematurely before the Royals' World Series run in 2015, he did receive a World Series ring.