Duke, 35, signed a one-year, $2.15 million deal with the Twins in December. This season, the veteran lefty has posted a 3.62 ERA with 39 strikeouts and 15 walks in 37 1/3 innings. Unsurprisingly, Duke has been tougher on left-handed hitters.
Duke will help bolster the Mariners’ bullpen which already includes two lefties in James Pazos and Roenies Elias. Both have pitched quite well this season. But Duke has 14 seasons of experience under his belt and can alleviate the workload down the stretch. Entering Monday’s action, the Mariners are 62-43, four games behind the first-place Astros in the AL West.
De Jong, 24, pitched in seven major league games last year but has spent 2018 with Double-A Arkansas. He has a 3.80 ERA with an 89/34 K/BB ratio in 120 2/3 innings across 21 starts. MLB Pipeline ranked De Jong as the No. 28 prospect in the Mariners’ system.
Costello, 22, was taken by the Mariners in the 31st round of the 2017 draft. This year, with Single-A Clinton, the 1B/3B has hit .266/.360/.486 with 16 home runs and 70 RBI in 406 plate appearances. Costello was not among the Mariners’ top-30 prospects, according to MLB Pipeline.