The Mariners Found a Couple of Paul Sewald Clones

On June 20, the Mariners designated Sergio Romo for assignment. They had just finished an 11-game homestand where they had gone 2–9, and their record had dipped to a season-low 10 games under .500. The next day, Seattle beat Oakland, 8–2, and has gone 24–6 since then, pushing its way into the middle of the AL Wild Card race. Romo wasn’t the only (or main) reason why Seattle had struggled up to that point in the season, but his 8.16 ERA and -0.7 WAR certainly didn’t help either. He does provide a convenient inflection point, though, to talk about how critical the Mariners’ bullpen has been to their play over the past month.

Here’s a table showing how Seattle’s relief corps performance before and after Romo’s departure:

Earlier this season, Mariners relievers had trouble converting an excellent strikeout-to-walk ratio into consistent success. Romo was the worst offender, but Diego Castillo (5.25), Andrés Muñoz (4.50), Drew Steckenrider (5.65), Anthony Misiewicz (4.61), and Matthew Festa (4.35) all had ERAs over four through June 19. Their biggest problem as a group was an outsized home run rate that pushed their FIP up to 4.17 even though their xFIP sat at 3.83. Along with Romo, the Mariners also found ways to get Steckenrider and Misiewicz off their roster, replacing the latter with Ryan Borucki in a trade on June 4 and designating the former for assignment on June 11. All told, Seattle churned through 18 different relievers to start the season, three of whom are no longer with the organization and another seven who were shuttled back to the minors.

A couple of those relievers who struggled early on in the season have been key members of the Mariners’ turnaround. Muñoz went 17 straight appearances without allowing a run before giving up two in last night’s game against the Astros. Castillo and Festa have both been lights out, with ERAs of 1.42 and 1.13, respectively, since June 20. But the one constant has been Paul Sewald. An unknown reliever prior to last year, he improved the shape of both his fastball and slider last year to become one of Seattle’s most valuable relievers. He hasn’t been quite as good this year; after approaching a strikeout rate of 40% last year, that’s fallen to just above 30% this year — still elite but not as dominant. Thankfully, he has a whole corps of relievers behind him who are now putting together fantastic seasons, with a few of them boasting repertoires that curiously look similar to Sewald’s.

First, let’s pull up the reliever leaderboards for strikeout-minus-walk rate this season:

Amazingly, there are four Mariners in the top 11! Muñoz is up there alongside Díaz and Helsley, two dominant All-Star relievers. A little further down, we see Murfee and Festa show up, with Swanson a few spots behind them. (Sewald’s 23.1% rate ranks 34th.) Michael Ajeto recently analyzed the adjustments Muñoz made for Baseball Prospectus, and Zach Mason took a look at what Swanson has done well over on Lookout Landing this week. I want to dig into what’s driving the success of Festa and Murfee.

The key to Sewald’s breakout last year was lowering his release point and throwing his fastball up in the zone more often to give the pitch a much flatter shape; its vertical approach angle (VAA) was among the best in baseball last year. He paired that deadly heater with a newly developed sweeping slider that benefitted from seam-shifted wake effects. Festa and Murfee possess plenty of traits that match Sewald, and I suspect it’s no coincidence.

Let’s begin by comparing their fastballs:

All three pitchers have lackluster velocity on their four-seamers, and they don’t have a ton of ride like you might expect. Like Sewald, both Festa and Murfee possess extremely flat fastballs, with VAAs well above average. The latter gets there the same way Sewald does, with a low release point and high locations in the zone. Festa doesn’t have as low a release as Sewald, but he makes up for it by generating a bit more carry and by locating the pitch even higher in the zone. The three fastballs aren’t carbon copies, but there are enough common characteristics that the Mariners had to have known they were selecting pitchers who could replicate the success of their bullpen ace.

The results speak for themselves:

Among all four-seam fastballs thrown at least 100 times, Festa’s 37% whiff rate is the fifth highest in baseball, a hair behind Díaz. Murfee’s heater isn’t as effective as the other two relievers; he thrives via his slider instead.

All three pitchers throw sweeping sliders with plenty of SSW, and like their fastballs, all three have common traits that mirror each other:

Sewald’s and Murfee’s sliders are true sweepers, with an extremely flat vertical approach angle and a wide horizontal approach angle as they cross the plate. Because Festa’s release point is a little higher than the other two, his slider doesn’t sweep as much, though it still breaks horizontally a ton. Again, none of the three pitches are exact matches, but there are enough commonalities between them to be more than a coincidence.

All three relievers possess whiff rates above league average for a slider, though Sewald’s stands above the other two. Both Festa and Murfee are able to induce swings a lot more often than their teammate, leading to more swings and misses overall.

So what’s the story behind these two unheralded relievers? Festa made his major league debut all the way back in 2018 and struggled to find any success across 30.2 innings split between ‘18 and ‘19. He was ranked 20th in our Mariners preseason prospect rankings in ’19, with a deep repertoire befitting a starter, but he underwent Tommy John surgery in early 2020 and missed more than a year rehabbing. This season, he came to spring training with a completely revamped pitch arsenal. The shape of his fastball looked the same as it had before, but he had lowered his release point by more than a quarter of an inch, giving the pitch its flatness.

Festa’s slider was the true revelation, though. Back in 2019, he was throwing it with 7.2 inches of horizontal break. It was a good pitch for him — probably his best secondary offering — but it really dazzled this spring, with over nine more inches of horizontal break than before. He made the major league roster out of spring training and struggled in his first taste of the majors in three years, posting a 5.25 ERA and a 6.46 FIP across 11 appearances before hitting the IL with a minor elbow injury. But since returning from that injury, he’s been phenomenal, allowing just four runs in 16.2 innings with a 1.38 FIP.

Murfee’s journey to the big leagues was just as unlikely as Festa’s. He was a 33rd-round draft pick back in 2018 and slowly made his way through Seattle’s farm system, working as a starter and reliever. Without overpowering stuff, he was often overlooked, though his unique release point and solid command gave him some promise. He was ranked 21st on our Mariners preseason prospect rankings this year, made his debut in late April, and has been a key member of Seattle’s relief corps ever since.

It should be no surprise the Mariners developed two pitchers with similar characteristics as one of their biggest development wins recently. Flat fastballs and sweeping sliders are trendy pitches right now because they’re so effective at generating both whiffs and weak contact. Festa should be commended for identifying those adjustments and implementing them after rehabbing from his injury, and the team should be commended for sticking with Murfee despite the non-traditional stuff and approach; it just happens that his strengths fit right into the trends of modern pitching design.

Amid this hot streak, the bullpen will be tested again, as the Mariners were forced to place Castillo on the IL on Thursday with shoulder inflammation. It sounds like it’s nothing serious, but with a series against the Astros this weekend and then six games against the Yankees in the next two weeks, they’ll need all hands on deck in the bullpen. Festa, Murfee, Muñoz, and Swanson will all be called upon to bridge the gap to Sewald in the ninth inning as Seattle continues its push in the AL Wild Card race.