The Mariners on Monday added right-handed reliever Arquimedes Caminero to the 25-man roster and optioned starter Taijuan Walker to Triple-A Tacoma.
Caminero, 29, was acquired by Seattle from Pittsburgh on Saturday in exchange for a player to be named later.
Walker, 23, was activated from the 15-day disabled list (tendinitis in his right foot) on Saturday and allowed six runs on seven hits in four innings against the Angels that night.
"I was kind of surprised," manager Scott Servais said after Walker's outing. "I was looking for more. I was expecting more.