Armstrong has been Tacoma's best overall reliever this season. Right-hander Nick Rumbelow was optioned to Class AA Arkansas

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SAN DIEGO — The Mariners swapped out bullpen pieces on Tuesday afternoon, bringing up the best performing reliever in Class AAA Tacoma to help out their bullpen.

Right-hander Shawn Armstrong had his minor league contract selected and was added to the active roster. To make room for Armstrong, right-hander Nick Rumbelow was optioned to Class AA Arkansas.

“It’s an opportunity to take a look at him,” manager Scott Servais said. “He was on the list of guys we definitely wanted to call back for September. It’s an opportunity to get to him in a little bit earlier. He’s had a really good season. He’ll help out in situations where we’ve been leaning heavily on Nick Vincent and Adam Warren.”

Armstrong, who turns 28 on Sept. 11, has been solid all season for the Rainiers. He’s 15-for-17 in save opportunities with a 2-5 recored and a 1.77 ERA (11 ER, 56.0 IP) in 49 relief appearances. He’s struck out 82 batters with just 26 walks.  In his last 26 outings, dating back to June 14, he has not allowed an earned run in 25 of them. He is 14-for-14 in save opportunities with a 0.66 ERA (2 ER, 27.1 IP) with 44 strikeouts and 10 walks while holding opponents to a .194 (18 for 93) average against in that span. Right-handed batters are hitting just .170 (18×106) with 48 strikeouts vs. Armstrong, while lefties are batting .217 (20×92) with 34 strikeouts.

“My cutter has probably been my best pitch,” he said. “I made a few mechanical adjustment and have been keeping that stuff in tact.”

The Mariners acquired Armstrong in the offseason from the Indians for international slot money. He seemed like a candidate for a middle relief spot, but struggled during spring training and dealt with some decreased velocity.

“He didn’t have a good spring,” Servais said. “The stuff was down. The velocity was down. He didn’t have a good feel for the curveball. It was disappointing. And he was disappointed with how his spring went. But he got a few things ironed out in Tacoma.”

Since he was out of minor league options, the Mariners designated Armstrong for assignment on March 7. He cleared waivers and was outrighted to Tacoma. The lack of minor league options is a reason why Armstrong was stuck in Tacoma for so long despite his successful results.

Rumbelow, 26, has a 6.75 ERA (10 ER, 13.1 IP) with nine strikeouts and three walks in nine relief appearances with the Mariners over three different call-ups.