"Hopefully we'll get him back relatively soon," Servais said. "He did the right thing the other day, alerting us it didn't feel right and get him out of the game before something was worse."
Phelps, 30, was one of the Mariners' most significant acquisitions prior to the non-waiver Trade Deadline, as general manager Jerry Dipoto sent four Minor Leaguers to the Marlins for the veteran right-hander on July 20.
Phelps posted a 2.57 ERA in seven innings over seven appearances for Seattle, but he was taken out of Sunday's 8-7 win over the Royals in the first game of a doubleheader after facing just one batter. For the season, Phelps is 3-5 with a 3.33 ERA in 51 games.
Phelps said his arm hasn't felt quite right in his past few outings, and he felt continued discomfort in his elbow after throwing seven pitches to Drew Butera before getting him to fly out to center leading off the seventh Sunday.
"The last couple times out, I just really haven't gotten loose like I have in the past," Phelps said. "As far as getting loose in the bullpen it's been fine, but I get in the game and it's still tight. I can tell the ball just isn't coming out, and my stuff hasn't really been the same."
Phelps spent time on the disabled list in 2013 with a strained forearm and in '14 with shoulder inflammation while with the Yankees. He was also on the DL in '15 with a stress fracture in his forearm and last year with a strained oblique while with the Marlins.
The move creates an opening for Alonso, who was acquired by trade from the A's on Sunday. The Mariners began a two-game series in Oakland on Tuesday night. It also leaves Servais with a seven-man bullpen, something that hasn't happened very often this year.
"We were going to the long reliever or the middle reliever earlier in ballgames. I think you might see us do that again when certain guys are on the mound," Servais said. "We're going to use those guys a lot, and it may be a situation where, if we do go through a long reliever, we may have to call a guy up from Tacoma. We did that earlier in the season and we were able to keep things together."
The Mariners added more bullpen depth in a trade Tuesday by acquiring right-hander Ernesto Frieri from the Rangers for cash considerations. Frieri, who has pitched in more than 300 Major League games in his career , will report to Triple-A Tacoma Wednesday and could be a factor for Seattle at some point this season.
"He's got Major League experience," Servais said, mentioning he knows Frieri from when he was with the Angels. "If we do start circulating guys up and down, he's a guy that's been there before and you can count on."