SEATTLE -- Mariners general manager Justin Hollander was optimistic on Friday afternoon about Matt Brash’s progress toward a return from right elbow inflammation, with the leverage reliever slated to begin a rehab assignment with Triple-A Tacoma as soon as Saturday.

But that optimism proved to be short-lived after Brash experienced another setback after throwing at T-Mobile Park ahead of Seattle’s series opener against the D-backs on Friday, Hollander revealed on Saturday.

Brash has been shut down indefinitely.

“He threw and did not feel good at all,” Hollander said. “I think his words were ‘Every throw felt bad.’ We met with our physician last night and we are concerned.”

Brash, who’s since returned to Arizona, is awaiting a follow-up appointment with Dr. Keith Meister in Texas, the renowned orthopedist with whom he initially met with in March.

Brash, who began feeling soreness in his elbow after two bullpen sessions in Spring Training, was placed on the 15-day injured list before Opening Day, but was told that he would not require surgery and he began ramping up in early April. However, while his stuff has been good, the Mariners have been concerned about how he was “bouncing back” after each session.

“It’s sort of been a pattern over time,” Hollander said. “His stuff is great when he goes out there, he’s touching 99 [mph], sitting 96, 97, 98. But each progressive outing, he has not bounced back and he’s bounced back slower each time.”

Those issues had already pushed Brash’s plans for a rehab assignment recently. The Mariners had initially intended for Brash to embark on one during their most recent road trip to Denver and Arlington.

“Obviously, we’re concerned. He’s concerned,” Hollander said. “But we’ll take it from there once Dr. Meister has weighed in.”

The right-hander led the Majors with 78 appearances in 2023, over which he carried a 3.06 ERA. His 34.7% strikeout rate and 38.1% whiff rate -- including a 48.5% whiff rate on his slider -- both ranked in the 98th percentile among pitchers, while his average fastball velocity of 98.1 mph put him in the 96th.

Brash stepped into the Mariners’ highest-leverage roles last year, along with Andrés Muñoz, after Seattle dealt Paul Sewald to the D-backs at the Trade Deadline. Brash was again expected to be a big part of the bullpen’s plans in 2024.

The Mariners have also been without Gregory Santos (right lat strain) since acquiring the high-velocity righty from the White Sox in January. Santos, who was transferred to the 60-day IL on Thursday, has begun throwing up to 120 feet and is expected to add more intensity soon. The Mariners are optimistic that he’ll be ready close to when he’s eligible to be activated, on May 24.

Despite the absences of Brash and Santos, Seattle’s relievers have thrived. They lead the Majors with a 2.40 ERA and rank eighth with a 24.9% strikeout rate.

Muñoz has been their top arm, with Ryne Stanek in the primary setup role. Gabe Speier has been their lefty linchpin, with one earned run and 15 strikeouts in 10 innings. But just about every reliever has seen success in whatever role deployed, notably Tayler Saucedo, Trent Thornton and Tyson Miller.

The Mariners have withstood Brash’s absence to this point, and at least for the foreseeable future, they’ll continue looking to fill the void.