Phelps, acquired last week for four Minor League prospects, made his Mariners debut in the seventh inning of Saturday's 6-5 victory over the Yankees, for whom Phelps started his career.
With Seattle nursing a 4-3 lead, Phelps retired the side in the seventh -- one left-handed hitter and two righties -- and then came back out in the eighth and fanned right-handed-hitting Ronald Torreyes.
"Last night, the reason I put him out there was that it was probably going to be an inning," Servais said. "But then you look at the lineup and where you're at, I thought maybe we could get one or two more out of him. It worked out great."
In 44 relief appearances with Miami this season, Phelps was 2-4 with a 3.45 ERA with 51 strikeouts in 47 innings. Left-handers hit .258 against him, and right-handers .225.
"He's pretty equal vs. the righties and the lefties, which is nice," Servais said the day of the trade. "You don't get so platoon-heavy or situational-heavy there."
Servais likes the flexibility that Phelps adds to the bullpen.
"I don't think there's a manager in the league that doesn't like those guys. It just makes things a little easier," Servais said. "Knowing that if they go one and change, or two innings, they're probably going to be down for a couple days. But, when you're in the course of those games and having to flip-flop, you know, this guy's better against righties, now who's your lefty specialist, now who's the righty behind him -- you can run through a bunch of guys."
"Certainly it gets challenging in a tie game, where all of a sudden you're extending too far, you're running out of pitching toward the end. So, those guys that can go multiple innings and handle both sides of the plate, both lefties and righties, are really valuable."
Although used primarily out of the bullpen the last two seasons, Phelps has 64 starts among his 219 games in the Majors, including 19 starts with the Marlins in 2015.
For the time being, however, Servais said Phelps will remain in a relief role.
"I like him in our bullpen right now. We'll see where he is down the road, but for the foreseeable future, he's going to be a key part of our bullpen," Servais said.
Jim Hoehn is a contributor to MLB.com based in Seattle. He covered the Mariners on Sunday.