Mateo split the season between the Double-A San Antonio Missions, the Triple-A Tacoma Rainiers, and the Major League Mariners. He went 1–0 with an 0.52 ERA in 12 outings for San Antonio and 4–2 with a 4.06 ERA and six saves in 20 appearances for Tacoma. ... He made his Major League Baseball debut on May 7, , for the Mariners against the Toronto Blue Jays. Mateo gave up 10 runs in 21 innings during his first taste of Major League action with Seattle.