Heading into the 2015 season, Sanchez was expected to pitch out of the bullpen, and to compete for the closer role. After Marcus Stroman suffered a torn left ACL on March 10, Sanchez pitched in the rotation for the rest of spring training and earned the fifth starter spot to open the season. He recorded his first win as a starter on April 22, defeating the Baltimore Orioles 4–2. Sanchez pitched 5 innings and yielded two runs on two hits and seven walks with four strikeouts. In winning, he became the first Toronto pitcher to win despite issuing seven walks since Jeff Ware in 1995. On May 8, Sanchez shutout the Boston Red Sox over seven innings, recording his first quality start. The Blue Jays would win the game 7–0. On June 5, he pitched into the ninth inning against the Houston Astros and took the win, 6–2. In addition to setting a career-high in innings pitched, Sanchez did not walk a batter for the first time as a starter. After missing his next scheduled start with upper body soreness, Sanchez was placed on the disabled list on June 14, retroactive to June 6, with a strained lat muscle. On July 19, manager John Gibbons announced that, upon his return from the disabled list, Sanchez would pitch out of the bullpen. He was activated on July 25 in Seattle. Sanchez was ejected from a game against the Kansas City Royals on August 2, for hitting Alcides Escobar with a pitch after home plate umpire Jim Wolf had issued a warning in the first inning. MLB deemed it intentional, and he was suspended for 3 games on August 4. Sanchez made 11 starts in the 2015 regular season and 30 relief appearances, earning a 7–6 record, 3.22 ERA, and 61 strikeouts in 92 innings. He appeared in all five games of the Division Series against the Texas Rangers, and did not allow a run in 5 innings pitched. In the Championship Series played against the Kansas City Royals, Sanchez made four scoreless relief appearances as the Blue Jays were eliminated from the postseason in six games.