Mariners at Memorial Day: Bullpen

Seattle's relievers have been a strength for the team. Is their current success sustainable?

Chris O’Meara / AP Images

We’ve already talked about the lineup and starting rotation in the first two segments of the Mariners at Memorial Day series. Let’s cap it off with a discussion about a group pivotal to the team’s chances of returning to the postseason - the bullpen.

Strong season numbers

Seattle’s relievers have been an ideal complement to arguably the best starting rotation in baseball. As you can see below, the bullpen ranks near-or-at the top of MLB in multiple categories.

An element we shouldn’t overlook when discussing the Mariners’ bullpen is the relatively low number of innings it’s had to cover thus far. This is directly attributable to a starting staff routinely providing length to manager Scott Servais.

Something to consider that’s not illustrated above. Seattle relievers have allowed fewer extra-base hits (40) than any bullpen. Having the second-highest ground ball rate (50.4%) in MLB certainly helps in this regard. Coincidentally, the team leading the majors - the Yankees (53.2%) - will be in the Emerald City on Memorial Day.

A bumpy road to success

Although a lot has gone right for the bullpen, there have been challenges to overcome. Andrés Muñoz, the team’s best reliever in 2022, has missed most of the season with a deltoid injury. Another extremely valuable contributor to last year’s reliever rotation, Penn Murfee, has been on the IL for almost a month with elbow inflammation.

Compounding matters, Diego Castillo and Matt Festa struggled so much in April that the duo was assigned to Class-AAA Tacoma’s bullpen. Castillo was outrighted and is no longer on the 40-man roster. Quite the descent for a pitcher used in high-leverage situations just one year ago.

Getting the job done

Despite these setbacks, the remaining group of relievers has performed remarkably well. In fact, my go-to metric - xwOBA - suggests the Mariners have one of the better bullpens in the majors for a third consecutive season.

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. The MLB average xwOBA this season = .324

A review of individual Seattle relievers reveals all but one member of the active bullpen has a better-than-average xwOBA.

xwOBA of Active M’s Relievers
Paul Sewald - .197
Gabe Speier - .244
Trevor Gott - .257
Justin Topa - .271
Tayler Saucedo - .268
Matt Brash - .285
Chris Flexen - .316
Juan Then - .352

MLB Average xwOBA = .324

xwOBA of Other Notable M’s Relievers
Penn Murfee** - .277
Andrés Muñoz** - .288
Diego Castillo* - .345
Matt Festa* - .467

* Class-AAA Tacoma
** Injured List

Leading the way, a rock for the Mariners since joining the club in May 2021 - Paul Sewald. The right-hander’s 35.6% strikeout rate is about six-percent higher than last season and ranks 14th-best among 242 relievers facing 50-plus batters. Sewald is also allowing significantly less hard contact and fewer walks. Therefore, it’s not surprising his .197 xwOBA is fourth-best within our group of 242 relievers.

By now, it should be clear to everyone the Mariners have developed a core competency of helping unheralded pitchers, like Sewald, become the best version of themselves. This winter’s batch of unexpectedly good additions are Justin Topa, Gabe Speier, and Tayler Saucedo. The trio has helped fill the void created by the unplanned losses we’ve discussed.

Another newcomer, eight-year veteran Trevor Gott, has also been central to the bullpen’s success. Gott’s 25.9% strikeout and 5.6% walk rates are significantly better than his career averages. Moreover, the Kentucky alum is one of four relievers to face at least 100 batters and not allow a home run. The others: Brent Suter (COL), Griffin Jax (MIN), and Kevin Kelly (TBR).

Sophomore reliever Matt Brash has experienced his share of ups and downs over the first two months of the season. Still, a comparison of his 2022 numbers as a reliever to this year reveals the Canadian is striking out more hitters and allowing fewer free passes. In fact, Brash’s 32% strikeout-to-walk ratio is fourth-best among relievers.

Brash’s Reliever Stats
2022 - 33.9 SO% | 12.6 BB%
2023 - 40.8 SO% | 8.7 BB%

Despite Brash boasting better strikeout and walk numbers, opponents have carved out a noticeably higher .289 AVG against him than last year (.202). So, what’s going on with the right-hander?

A contributor to the jump in AVG is Brash’s success, or lack thereof, on ground balls. Entering today, opponents had a .478 AVG on grounders allowed by the 25-year-old - highest among 160 relievers surrendering 20-plus grounders.

Taking it a step further, Brash has allowed 10 hard-hit ground balls with seven becoming at hit for a .700 AVG. Only three of 234 pitchers allowing at least 10 hard-hit grounders have experienced worse results. With this in mind, it’s reasonable to expect the Niagara product experiences better ground ball success as his season evolves.

Initially assigned to the Opening Day bullpen, Chris Flexen was thrust back into the rotation after the seasons’ second game to replace an injured Robbie Ray. Flexen would make four starts, which were a mixture of good and bad, before becoming a reliever again.

The sample sizes are small, but Flexen’s stats do look more favorable when he’s pitched out of the bullpen. Again, there’s not enough data to make a definitive conclusion.

This isn’t meant to be a criticism of Servais or the team’s other key decision-makers. But it’s hard for this uninformed nerd to grasp what Flexen’s actual role with the Mariners is. He’s appeared sparingly - just six games since returning to the bullpen on April 23. Moreover, the club was down by three-plus runs when the Newark, CA native entered five of those six contests.

It’s a bit early to assess rookie Juan Then considering he’s made just eight big-league appearances. That said, one thing did catch my eye: Then’s low strikeout rate. He’s struck out just four hitters in 8.2 innings. The right-hander averaged 11 SO/9 with Class-AA Arkansas before his call-up.

Heavy lifting?

I’m not a pitching expert; that’s been established by now. Having said that, I do believe the absences of Muñoz, Castillo, and Murfee have pressurized the bullpen by leading to heavier-than-preferred workloads for the team’s current group of relievers.

Appearance by Key M’s Relievers
Matt Brash - 27
Trevor Gott - 26
Gabe Speier - 25
Paul Sewald - 24
Justin Topa - 23

Just 12 clubs have at least one reliever with 25 appearances. Seattle is the only one to have three. Only Arizona (four) has more than the Mariners. Perhaps this isn’t a big deal, but it feels like it is to me.

Brash has literally pitched in over half of his team’s 53 games this season. Granted, the 25-year-old has faced just one batter in each of his last two appearances. Overall, he’s logged 21.2 innings in 27 contests.

Obviously, the Mariners’ brain trust knows infinitely more about managing a bullpen and its supporting cast than me. But Brash, Topa, and Speier are currently on a pace to blow past their professional highs for appearances in a season. Sewald and Gott are also on a trajectory to set personal bests as major-leaguers.

Seeing the mounting number of appearances by these five key relievers does leave me wondering whether the bullpen, as it’s currently configured, can remain outstanding for the entire season.

There’s another side effect of not having Muñoz, Castillo, and Murfee to consider. The team is using Sewald almost exclusively as a ninth-inning closer. Entering today, 20 of the San Diego alum’s 24 appearances have been in the ninth inning. Last season, he pitched in the ninth-or-later in just 10 of his first 24 games.

Reinforcements on the way?

There are several internal options, who could potentially help bolster the bullpen. The timing of their availability hinges on two factors - health and/or effectiveness.

Clearly, a healthy Muñoz would be a big boost for the backend of the bullpen. His presence would make it easier for Servais to employ the matchup strategy he’s so skillfully used over the previous two seasons.

Murfee’s return would also mitigate reliever workload, while improving the bullpen’s reliability and stability. The Santa Clara product’s opposing hitter AVG and SLG were top-10 among MLB relievers prior to his injury. Furthermore, Murfee led Seattle relievers with 67.1 innings pitched last year.

Still, we shouldn’t get ahead of ourselves until Muñoz and Murfee actually rejoin the big-league club. Both pitchers have yet to begin minor-league rehab assignments. Is this a development worth being concerned about?

Perhaps, but it’s also plausible management is simply being cautious with the recovery of two extremely valuable relievers.

Everyone’s new favorite shiny penny, Prelander Berroa, could join the Mariners within a matter of weeks. His call-up from Arkansas likely depends on how he progresses through the team-established plan designed to prepare him to be a big-league reliever. In 8.1 innings spanning six relief appearances, the right-hander has held opposing hitters to a .039 AVG with 15 strikeouts and five walks.

Since joining the Rainiers in early April, Festa has allowed seven hits and one home run in 21 innings, which looks good from afar. Furthermore, the right-hander has surrendered just four walks in his last 16 innings. He had allowed six bases on balls in three games with the Mariners before his demotion to the minors.

Castillo has held opponents to a .200 AVG in nine minor-league appearances. But he is struggling with surrendering free passes. The right-hander has allowed 10 walks to go with nine strikeouts. In fact, the six-year veteran has walked at least one batter in each of his outings.

Trouble on the horizon?

So much seems to hinge upon the return of Muñoz and Murfee. But what if either suffers a rehab setback or doesn’t come back this season? These are realistic scenarios. Then what?

Perhaps Berroa becomes the next Edwin Díaz or evolves into another high-leverage arm for Servais to call upon. But those are lofty expectations for an untested 23-year-old.

Festa likely gets another opportunity with the Mariners at some point. But how much will his presence move the meter?

And what if a current reliever on the big-league roster suffers an injury or regresses?

Losing Sewald for any period would be devastating. Not having Brash or Gott would also be a big blow. So would any combination of Gott, Topa, Speier, and Saucedo taking a step back from their early-season effectiveness.

Yes, I’m being a dark cloud. But to me, it wouldn’t take much for the current bullpen to have a greater negative effect on Seattle’s chances of returning to the postseason this year than its highly-scrutinized lineup.

My Oh My…