The Mariners are expected to bring up flame-throwing reliever Edwin Diaz from Class AA to join their bullpen. Diaz was the Mariners' minor league starting pitcher of the year in 2014 and 2015, but was converted to a relief role in May.

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ARLINGTON, Texas — The roster move hasn’t been announced, but baseball sources confirmed that reliever Edwin Diaz, the Mariners’ top pitching prospect in the organization, will have his contract selected from Class AA Jackson and be with the team in Texas for tonight’s game against the Rangers.

Reliever, Cody Martin, who pitched three innings in the Mariners’ 7-4 loss to the Rangers on Friday night and an inning on Thursday is expected to be optioned back to Tacoma.

With Joel Peralta being designated for assignment, Seattle had an open spot on the 40-man roster for Diaz.

Diaz was named the Mariners’ minor league starting pitcher of the year in 2014 and 2015. But in early May, the organization began the process of converting Diaz to a reliever.

“The guy has a chance to be a pretty good reliever,” general manager Jerry Dipoto said about the process in May. “Edwin has plus fastball velocity, plus fastball life and what at times is a plus breaking ball. The changeup has never really been easy for him. It’s not the most conventional arm action and delivery you will ever see. So the likelihood of him picking up that third pitch was maybe a little bit more remote.”

The belief was that Diaz would flourish in the role because of his power fastball and solid slider. And that it would also work well with a smaller body frame. The Mariners lack power relief arms in the organization.

“We felt like as a starter he’s mostly 90-94 mph with a good slider,” Dipoto said. “What happens when we dump him into a bullpen? Do we have a chance to get that back-end dynamic arm? And does that supersede the ceiling that he would have as a starter? As nice as his progression has been as a starter, we felt like his ceiling was probably more back of the (starting) rotation because of the third pitch and the general wonder about how he could handle the innings because it’s not the largest frame among the pitchers in our system. But as a reliever, if he can acclimate himself to the bounce back and show the resiliency to be a bullpen guy, does the stuff tick up?”

The results have been better than expected. Diaz dominated in the role, showing a fastball that consistently sat at 96-98 mph and touched 99 mph. In 10 relief appearances, he’s allowed one unearned run  in 11 2/3 innings pitched with three hits, two walks and 16 strikeouts. Diaz’s early outings were scripted in their duration and rest in between. But in recent weeks he’s pitched without a script and continued to perform at a high level. The early and easy success wasn’t anticipated.

“What’s a word to explain that?” said Andy McKay, Mariners director of player development. “It’s overwhelming. I’m very happy with it. Tons of strikes with his fastball and slider. He’s been totally overmatching hitters since he’s gone into that role.”