Mariners Acquire RHP Cooper Criswell from Mets in Trade

The Seattle Mariners just made a subtle, maybe even sneaky, roster move by bringing in right-handed pitcher Cooper Criswell from the New York Mets for cash. It’s not a blockbuster, but it shows Seattle’s focus on adding pitching depth and tweaking the 40-man roster as the season approaches.

This deal has implications for both the big-league club and Triple-A. It gives us a glimpse into how the Mariners are shaping their pitching staff for the future.

Mariners Add Pitching Depth With Cooper Criswell

At first glance, Cooper Criswell looks like a classic low-risk, depth move. But for a team that’s always hunting for stability and flexibility on the mound, he’s a steady option who’s already weathered the ups and downs of MLB life.

Criswell, 29, heads to Seattle after a short but respectable stint with the Boston Red Sox in 2025. He pitched in seven games, put up a 1-0 record with a 3.57 ERA, and looked pretty composed in those limited chances.

He’s not exactly a flamethrower, but he’s managed to keep his teams in games. That’s something every manager quietly values during the grind of a long season.

From Boston to New York to Seattle

Criswell’s journey to the Mariners has been anything but straightforward. He started the offseason with Boston, then the Mets claimed him off waivers on December 5.

New York designated him for assignment earlier this week, and Seattle saw their opening. This kind of roster shuffle is just part of modern baseball, especially for pitchers bouncing between the majors and Triple-A.

For the Mariners, grabbing Criswell meant adding a pitcher with recent MLB experience—no need to give up prospects or anything long-term.

Roster Move: Jhonathan Diaz Designated for Assignment

To clear space for Criswell, Seattle designated lefty Jhonathan Diaz for assignment. Diaz didn’t make a huge impact at the major-league level in 2025, but he’s been around the organization for a while.

Diaz, also 29, only got into one big-league game for Seattle. He spent most of his season with Triple-A Tacoma, taking on a heavy workload and anchoring the Rainiers’ rotation.

Strong Triple-A Production in Tacoma

Even with limited MLB time, Diaz was one of the Pacific Coast League’s more productive pitchers. He pitched in 27 games, put together an 11-6 record, and racked up some serious innings for Tacoma.

He actually led the Pacific Coast League with a 4.15 ERA. That says a lot about how tough that league can be for pitchers, and about his durability. The ERA isn’t eye-popping, but being steady and available matters, especially in Triple-A.

What This Means for Seattle’s Pitching Staff

Adding Criswell gives Seattle another right-handed option as they juggle their pitching staff. The Mariners have made depth a priority, knowing full well that injuries and slumps are just part of the deal.

Criswell fits right into that approach. He could be:

Rotation depth for Triple-A Tacoma

A spot starter or long-relief arm in the majors

Insurance if injuries start piling up

With Diaz out, the roster balance shifts a bit between lefties and righties. It’s a small adjustment, but sometimes those add up over a season.

Low-Risk Move With Potential Upside

Honestly, this move is more about flexibility than anything flashy. The Mariners opened up a 40-man roster spot and picked up a pitcher who’s shown he can handle the majors, all for just some cash.

Moves like this slip under the radar in March but can mean a lot by July. If Criswell gives them reliable innings or steps up when things get dicey, the front office will look pretty sharp for jumping in early.

 
Here is the source article for this story: Mariner pick up RHP Criswell in trade with Mets