Servais said Zunino's issues are different than in the past and he needs to fix a mechanical issue in his swing more than his mental approach.
"When that's happening at the big-league level, it's really hard to make major changes here," he said. "We thought let's take the foot off the gas a little, get him down to Tacoma and get him right. And as soon as we get him right, he'll be back. He's not down there for an extended period, but we do need him right."
Gosewisch was hitting .240 with four doubles and seven RBIs in 17 games for Tacoma. The 33-year-old was claimed off waivers from the Braves in January after spending parts of four seasons with the D-backs from 2013-16, where he hit .199 with five home runs and 30 RBIs in 126 games.
Zunino started 21 of Seattle's first 29 games and came off the bench in three others, but was hitting just .167/.250/.236 with five doubles and two RBIs in 72 at-bats.
Carlos Ruiz, acquired by the Mariners as a 38-year-old backup to Zunino, will now share time with Gosewisch while Zunino works to get back on track in Tacoma.
"That's the plan," Servais said. "We'll go with both those guys. They certainly don't have all the upside that Mike brings, but they're good ballplayers and can catch and throw and put the bat on the ball. So we'll go with that for a while." Gosewisch made his Mariners debut in Friday's loss to Texas, playing the final four innings after Ruiz was pinch hit for in the ninth. He flew out to center in the 11th in his lone at-bat.
Zunino hit .207 last year with 12 homers and 31 RBIs in 55 games, but hiked his on-base and slugging percentages to career highs of .318 and .470 after spending the first half of the season in Tacoma.
But he couldn't match that improvement this year and had struck out 30 times with six walks and shown none of the power from earlier in his career, when he hit 50 homers in 185 games from 2013-16 after being selected with the third overall pick in the 2012 Draft out of Florida.
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.