Aro and left-hander David Rollins were called up from Triple-A Tacoma on Friday while Wade Miley was placed on the 15-day disabled list with a left shoulder impingement and Steve Johnson was designated for assignment.
Rollins pitched 1 1/3 innings in Friday's 8-4 win and then was optioned back to Tacoma on Saturday when starter Adrian Sampson was activated. That was the plan all along, with Aro now taking a longer-term spot in the bullpen as a versatile right-hander who can provide multiple innings if needed.
That's a key element, given the uncertainty facing Sampson with his MLB debut and Sunday starter Taijuan Walker dealing with a sore arch that shortened his last outing to 3 1/3 innings.
"When the dust settles here, it's Aro for Steve Johnson," manager Scott Servais said. "Just his ability to maybe go a little longer, multiple innings, a little bit different type of pitcher. He can throw a little harder, the offspeed is a little different than what Steve threw up there. We just thought it was a better fit for us right now, and he's fresher."
Aro, acquired along with Miley in exchange for Carson Smith and Roenis Elias, was 2-2 with a 2.45 ERA in 33 innings over 22 appearances in Tacoma. He's the only player in that deal currently on an active big league roster, as Miley and Smith are on the disabled list and Elias was sent back to Triple-A Pawtucket by the Red Sox after taking the loss in Friday's game.
The Mariners like Aro's upside and are interested to see how he fits in after a disappointing Spring Training.
"He wasn't the same guy in the spring that I saw in winter ball, but I think we've got him back on track," said Servais. "On a new team with a different set of eyes, you're trying to impress people, but you need to be who you are. I think he's back to doing that. The reports have been very good."
Aro said he's focused more on just throwing strikes in Tacoma and controlling counts. As for fitting in better now?
"Obviously I had a lot of friends in Boston," he said through translator Fernando Alcala. "But now that I've been here, I feel really comfortable. It feels like this is family."
If Johnson clears waivers, he could be back in Triple-A Tacoma soon, but there's no guarantee that will happen.
"I would love to have Steve back, and we expressed that to him," Servais said. "But I do think there's a relatively good chance he gets claimed."
There's a much higher likelihood that Rollins returns at some point. He has Minor League options, so the Mariners merely optioned him back to Tacoma. But he's impressed the organization with how he's thrown all season, including Friday's appearance.
"He certainly showed well," Servais said. "And as you're scouring, looking around for bullpen depth, it's nice to see them come up and you can say, 'Yeah, that's a guy we can count on.' He's earned that right. It certainly won't be the last time you see him up here this year."