"I really liked what Rollins was able to do, really come in and pound the strike zone," manager Scott Servais. "And that's been the difference for him. That was the report in Triple-A. He just goes right after them."
The club was able to add the two relievers for Friday's series opener at Fenway Park after placing starter Wade Miley on the 15-day disabled list, though it will move one of those back to Tacoma on Saturday when right-hander Adrian Sampson is expected to be activated to take Miley's next scheduled start.
The Mariners now have 10 days to trade, release or outright the contract of Johnson to the Minors. He was 1-0 with a 4.32 ERA in 16 2/3 innings over 16 relief appearances after being called up on May 3.
"Steve threw the ball well in spots," Servais said. "The guys we brought up, I feel better about their length opportunities in our bullpen. Aro has gone multiple innings a couple times and even started a game down there because they were short one day. Rollins being left-handed helps. Our bullpen is just beat up right now. We thought getting a couple fresh arms in here would help."
The bullpen had to absorb 7 1/3 innings in a 13-inning game on Wednesday against the Rays, plus extra work when Taijuan Walker was pulled after 3 1/3 frames on Tuesday with a sore arch. With Felix Hernandez and now Miley on the DL and Walker's situation a little tenuous, Aro provides a long-relief option.
Aro, 25, was 2-2 with one save and a 2.45 ERA with 24 strikeouts in 33 innings over 22 games with Tacoma after being acquired from Boston as part of the same deal that brought Miley to Seattle in exchange for Roenis Elias and Carson Smith.
Rollins, 26, is 2-0 with a 2.36 ERA in 26 2/3 innings in 22 games with Tacoma this season. He made his Major League debut with Seattle last year after being acquired from the Astros in the Rule 5 Draft, going 0-2 with a 7.56 ERA in 20 appearances.
With Mike Montgomery and Vidal Nuno struggling to maintain their early-season dominance of late, Rollins adds another lefty to the mix, at least momentarily. The Texas native has 22 strikeouts and no walks in 26 2/3 innings for the Rainiers.
"I'm excited to get the opportunity to come back," said Rollins. "I've been patiently waiting on the phone call and got it yesterday, so I was pretty pumped about it."
The outstanding strikeout-to-walk ratio fits what the Mariners have been looking for from all their pitchers under the new regime.
"This year has been a little different," Rollins said. "I've just had the mindset of coming out and attacking guys and not giving up free bases. You can't defend a walk, so the less people I've got on base makes it a little easier to pitch."