KANSAS CITY -- Justin Dunn started his long-awaited rehab assignment on Thursday night with Triple-A Tacoma, the first steps toward a return to the mound for the Mariners after he had been sidelined since June 17 with right shoulder inflammation.

Unfortunately for Dunn and Seattle, though, the right-hander's outing ended before it could really begin.

Dunn threw just four pitches in his start with the Rainiers, exiting in the first inning after feeling some discomfort. Mariners manager Scott Servais confirmed as much before Friday’s series opener against the Royals at Kauffman Stadium, and suggested that it likely means the end of Dunn’s 2021 campaign.

"He didn't feel well, wasn't comfortable throwing so he came out of that ballgame and off the mound,” Servais said. “He'll get checked out, probably have another MRI, and certainly it looks like his season will come to an end."

Mariners general manager Jerry Dipoto said in a text message to MLB.com’s Daniel Kramer that, to date, there have been no indications of structural issues, though results had not yet come back on an MRI.

“We will reassess and update his plan once we’ve received current information,” Dipoto said.

Dunn was off to a solid start this season, not allowing more than three runs in any of his first nine starts, but he was placed on the injured list on June 2. Dunn made his return nine days later, but after just two shaky outings, he was once again placed on the IL with shoulder inflammation and hadn’t appeared in a game since.

The Mariners were eyeing a bullpen role for Dunn upon his return as they vie for an American League Wild Card spot, and will instead rely on their current crop of relievers, including Justus Sheffield, who also recently transitioned to a bullpen role after returning from the IL. Seattle entered Friday four games back of a Wild Card spot, trailing the Blue Jays, Red Sox, Yankees and A’s.

Fresh legs

The Mariners had a much-needed day off on Thursday after playing 12 games in 13 days, including a series with major playoff ramifications against Boston.

"Days off at this time of year are incredibly valuable, whether you've had an intense series or not,” Servais said.

Servais on Salvy

Royals catcher Salvador Perez dealt his fair share of damage to the Mariners when the teams previously faced off at the end of August. The All-Star hasn’t slowed since. On Thursday, Perez hit his 45th homer of the season, tying Johnny Bench for the most in a season by a primary catcher.

Servais, who caught for 11 seasons in the Majors, knows how impressive Perez’s offensive success is given the grind that comes with playing behind the plate.

"It's very impressive, there's no question,” Servais said. “And obviously having played that position my whole career, and knowing what that does to you and the toll it takes on you physically, to see what he's doing offensively is historic. It's an historic year for a catcher. I just hope we can slow it down for three days. … I say all those things, he wears the other uniform. And we got to get this sucker out the next couple days.”

Servais has had respect for Perez for a long time, having noticed his potential early on in his career -- even if he may have missed some of it.

“I remember seeing Salvy before he even made it to the big leagues,” Servais said. “I remember him on the back fields in Kansas City and saying, 'Who's this guy?' And thinking, 'Oh, he had a really good arm, he might be a decent catcher. Don't know if he'll ever hit.' How many times did we say that, and you look up now, that's probably 12, 13 years ago I'm talking, but he's had some kind of career.”