The award is given to the top reliever in the American League. Diaz was honored before Game 4 of the World Series in Los Angeles on Saturday night.
After a brilliant season where he led all of baseball with 57 saves — most of them protecting the slimmest of leads — Mariners closer Edwin Diaz has earned the top relief pitching honor.
Before Game 4 of the World Series in Los Angeles, Major League Baseball announced that Diaz has been named the 2018 Mariano Rivera American League Reliever of the Year, which is given to the season’s top reliever in the American League.
 The award was introduced by MLB in 2013 after Rivera’s retirement. Past winners include: Craig Kimbrel (2017 with the Red Sox), Zach Britton (2016 with the Orioles), Andrew Miller (2015 with the Yankees) and Greg Holland (2014 with the Royals).
Diaz, 24, converted 57 of 61 (93.4 percent) save opportunities, while posting an earned-run average of 1.96 (16 ER, 73 1/3 IP) with 124 strikeouts and 17 walks in 73 relief appearances with the Mariners in 2018. His 57 saves tied Bobby Thigpen’s total in 1990 with White Sox, which is the second most in MLB history. Francisco Rodriguez, who racked up 62 saves in 2008 with the Angels, holds the MLB record. Diaz became the second pitcher to record at least 50 saves and 100 strikeouts in a season, joining the Dodgers’ Eric Gagne, who accomplished the feat in 2002 and 2003 with the Dodgers, including his Cy Young season of 2003.
Of Diaz’s 57 saves, 27 came when he entered the game with a one-run lead, which is an MLB record, surpassing Gagne’s total of 24 one-run saves in 2003. The Mariners went 66-0 in 2018 when Diaz entered the game with a lead, including 61-0 in his save opportunities and 30-0 when he entered with a one-run lead.
Diaz was named to his first career All-Star team this season. He also led American League relievers in WHIP (0.79), while ranking second in strikeouts (124) and opponents’ on-base percentage (.229) and third in strikeouts per nine innings pitched (15.22) and ERA (1.96). He struck out 124 of the 280 batters he faced for a 44.3 strikeout percentage, which was the third-best percentage in AL history.  Â
He also reached numerous milestones this season, including his 100th career save and his 300th career strikeout. At 24 years, 151 days of age, he became the second-youngest pitcher to reach 100 saves since it became an official stat in 1969. Roberto Osuna reached 100 saves at 23 years, 62 days.
In three seasons with Seattle, DÃaz is 109 for 121 (90.1 percent) in save opportunities with a 2.64 ERA (56 ER, 191 IP) with 301 strikeouts and 64 walks in 188 relief appearances.