a 2.28 ERA on the year and narrowed his platoon splits versus left-handed hitting. He credited veteran teammate Matt Belisle and the Nationals' new pitching coach, Mike Maddux, for helping him improve his craft, while Maddux described Treinen's progress over the course of the season as "puppy dog to bulldog". Treinen led the National League in groundballs induced with 65.9% on the season and ranked highly in inherited runners stranded and soft contact created, among other metrics. Treinen was frequently used in situations when another Nationals pitcher had allowed one or more baserunners and manager Dusty Baker was seeking a double play ball, a role in which he excelled with his high-90s sinker. He remained with the Nationals for the entire year, working exclusively out of the bullpen, and made his first playoff appearance for the team, being credited with the win in Game 2 of the Division Series against the Los Angeles Dodgers and taking the loss in Game 4. With the departure of Mark Melancon to free agency, the Nationals named Treinen their closer for the 2017 season. On Opening Day, Treinen pitched a perfect inning in the top of the ninth against the Miami Marlins with two strikeouts to end the game, earning the save, the second of his career. The Opening Day performance was the first and last clean inning Treinen pitched in April. He gave up an earned run while notching his second save of the season on April 5 against the Marlins before blowing a save against the same team the following day. After just a couple of weeks as the closer in which he posted a 7.11 ERA, Treinen was removed from the role in favor of Shawn Kelley and later Koda Glover. In June, Treinen's batting average on balls in play against him gradually regressed toward his career norms. But with both Kelley and Glover on the disabled list, fellow setup men Enny Romero and Matt Albers unavailable, and acting manager Chris Speier in need of a reliever to close out the ninth inning of a 4–2 game against the Chicago Cubs on June 29, Treinen was called upon for his first save opportunity since being demoted from the position of closer. He gave up three earned runs for a blown save as the Cubs came back to win 5–4. Second stint with Athletics (2017–2019) On July 16, 2017, the Nationals traded