TOKYO -- An already thin Mariners bullpen has taken another hit as right-hander Shawn Armstrong, who was expected to be one of manager Scott Servais’ late-inning setup men, injured an oblique muscle in his right side and will be placed on the 10-day injured list to open the season.

Armstrong strained his right oblique in his final Cactus League outing last Wednesday against the Giants, but the injury wasn’t diagnosed until after the team flew to Japan on Thursday. Armstrong didn't pitch in either exhibition game in the Tokyo Dome, where the Mariners open their regular season on Wednesday against the A’s.

The club already is without Gerson Bautista, another hard-throwing right-hander, as well as veterans Anthony Swarzak and Sam Tuivailala, who all will open the season on the IL.

Third baseman Kyle Seager is also out for the first 10-12 weeks of the season following left hand surgery. Center fielder Mallex Smith won’t be placed on the IL, as he’s playing Minor League games in Arizona and hopes to be ready for the home opener in Seattle on March 28 after missing most of camp with a right elbow strain.

The bullpen wasn’t an area where the Mariners could afford to lose further depth, as the club already was looking to replace a heavy offseason of turnover after closer Edwin Diaz and setup men Alex Colome, Juan Nicasio and James Pazos were traded. Nick Vincent, Zach Duke, Adam Warren and David Phelps all left via free agency as well.

Armstrong, acquired from the Indians prior to the 2018 season, posted a 1.23 ERA in 14 appearances in the final month of the season after putting up a 1.77 ERA in 49 outings for Triple-A Tacoma. The 28-year-old was expected to fill a much larger role in the rebuilt bullpen this year, but those plans will now be put on hold.

While veteran free-agent addition Hunter Strickland fills the closer role, the rest of the Mariners bullpen will need to sort itself out as the season gets underway. Returnees Dan Altavilla, Chasen Bradford, Nick Rumbelow and Matt Festa are joined by free-agent signee Cory Gearrin and Rule 5 Draft pickup Brandon Brennan as the right-handers, with Zac Rosscup and Roenis Elias filling the lefty slots.