Mariners Designate Jhonathan Diaz, Select Kade Anderson

Kade Anderson is officially in the big leagues, as Thursday’s report of the star prospect’s impending promotion has given way to today’s announcement that the Mariners have selected Anderson’s contract.  To create room on both the 26-man and 40-man rosters, Seattle designated left-hander Jhonathan Diaz for assignment.

Diaz was himself selected to the Mariners’ active roster just three days ago, and his cup-of-coffee stint resulted in one appearance — one earned run allowed in 1 1/3 innings of relief work in the Mariners’ 7-4 loss to the Brewers on Thursday, as Diaz served up a homer to Garrett Mitchell.  This marks Diaz’s only bit of big league work this season, as he has otherwise spent 2026 with Triple-A Tacoma, posting a 6.22 ERA, 16.5% strikeout rate, and 8.3% walk rate over 111 1/3 innings.

Beyond those unspectacular numbers, Diaz has a 43% grounder rate at the Triple-A level this year, after spending most of his minor league career closer to the 50% threshold.  Diaz has also allowed 20 home runs, continuing his increased struggles at keeping the ball in the park.  The southpaw has allowed 38 homers over his last two seasons at the Triple-A level, after giving up 48 homers in his previous nine minor league campaigns.

Thursday’s outing officially made Diaz a veteran of the last six MLB seasons, though he has made only 18 total appearances and a 4.72 ERA in his 47 2/3 innings in the Show.  He has primarily worked as a starter in the minors but Diaz has been a reliever in 11 of his 18 big league games, and seems tabbed for long relief or swingman roles for any future looks in the majors.

If Diaz clears waivers, he can opt for free agency rather than accept an outright assignment to Triple-A, since he has been outrighted in the past.  This is now the third time Seattle has DFA’ed Diaz since he originally joined the organization in February 2024, and Diaz has opted for free agency both times before soon re-signing on a new minor league deal.  With relatively little time remaining in the 2026 season, Diaz might choose free agency and then re-sign again, or he could just accept the outright assignment to avoid any extra steps.

Anderson will make his MLB debut in today’s start against the Cubs, and Adam Jude of the Seattle Times reports that the Mariners will adopt a six-man rotation to fit Anderson into the pitching staff.  This plan may hinge on the availability of Bryan Woo and Bryce Miller, who have each been dealing with some recent health issues.  Miller has been trying to pitch through a tired arm, while Woo has a sore shoulder that pushed back his planned start today (opening the door for Anderson’s promotion).