Mariners add needed veteran reliever
Time will reveal whether adding Seranthony Domínguez was an impactful move for Seattle. But the metrics aren't encouraging.
The Mariners made their first move of the summer trade season by sending starting pitcher Luis Castillo to the White Sox in exchange for a much-needed veteran relief arm - Seranthony Domínguez. Chicago also included outfielder Nolan Jones and minor-league catcher Boston Smith in the deal.
This season, Domínguez has been effective at missing bats. Through the end of July, his 27.8-percent strikeout rate ranked 35th best among 166 relievers with 33+ innings. The only Mariner better than the team’s newest reliever is closer Andrés Muñoz (36.2-percent). That said, Domínguez has been surrendering a lot of free passes.
Only six of those 166 relief pitchers just mentioned have a higher walk rate than Domínguez. The Dominican Republic native experienced a similar challenge last year with a 13.8-percent walk rate.
Entering August, the right-handed throwing Domínguez had held righty bats to a .186 AVG, which was 41st best among 231 right-handers. That said, left-handed bats boast a .358 OBP and .424 SLG against him.
Domínguez’s .368 opponent SLG, which is noticeably better-than-average for relievers, may seem encouraging. However, opposing hitters have a 3.7-percent home run rate that’s on the wrong side of the league standard. Moreover, his 9.9-percent barrel rate falls in the bottom 20.
Initial reports have suggested that Domínguez will be slotted into a high-leverage role with the Mariners. The 31-year-old saved 12 games in 17 tries for Chicago this year. But lately, he’s been used in lower-leverage situations that some might refer to as “mop-up duty.”
And consider this. Domínguez’s xwOBA doesn’t stand up particularly well when compared to his new Seattle bullpen-mates.
Expected Weighted On-Base Average (xwOBA) uses quality of contact (exit velocity and launch angle) to determine what should’ve happened to batted balls. A key advantage to xwOBA is defense (good or bad) doesn’t influence it. This gives us a truer sense of how a hitter or pitcher is performing. MLB league-average xwOBA for relievers = .312
If we include Domínguez among current and injured Mariner relievers who’ve faced at least 75 batters this season, his xwOBA ranks eight out of 10. Not an encouraging sign for this nerd, who views the metric as an excellent tool for gauging pitcher performance.
SEA Reliever xwOBA (2026)
José A Ferrer (.286)
Cooper Criswell (.286)
Andrés Muñoz (.287)
Michael Rucker (.289)
Gabe Speier (.295)
Alex Hoppe (.296)
Eduard Bazardo (.311)
Seranthony Domínguez (.332)
Nick Davila (.336)
Cole Wilcox (.357)
Am I suggesting Domínguez won’t help the Mariners’ bullpen? No. But I’m taking a wait-and-see approach on whether he flourishes in a high-leverage role with his new club.
The other guys
Jones appears to be organizational outfield depth. The 28-year-old last played in the majors with Cleveland in 2025 when he posted a .211 AVG, .296 OBP, .304 SLG, and a 28-percent strikeout rate in 403 plate appearances. This year, he has a .231 AVG, .329 OBP, and .423 SLG with a 24.8-percent strikeout rate at AAA.
Smith, a left-handed hitting catcher, was acquired with an eye towards the future. MLB Pipeline rates the 23-year-old as Seattle’s 14th highest prospect.
Looking ahead
I’m interested to see what the Mariners do between now and Monday afternoon’s trade deadline.
Does the club deal other veterans, such as Randy Arozarena or even George Kirby?
Will it straddle the line by strategically buying and selling in an attempt to remain in contention, while adding for the future?
Do the Mariners reinvest Castillo’s current and future salaries, which the White Sox are covering, back into the 2026 and 2027 big-league rosters?
Finally
Perhaps Domínguez will make a meaningful impact this year. It’s worth noting he’ll remain under club control through the end of the 2027 campaign. That said, the numbers this blogger gravitates towards suggest Seattle added a middle-inning reliever today. Potentially a good thing, but not a season-altering move for a club in dire need of one.
My Oh My…