Did Mariners just get MLB’s next breakout reliever in Gregory Santos?

The idea that the Seattle Mariners may have pulled off a heist in their trade for reliever Gregory Santos has picked up steam ever since the move happened on Saturday.

Seattle Mariners get Santos from White Sox in trade for prospects, pick

Seattle dealt a pair of prospects – pitcher Prelander Berroa and outfielder Zach DeLoach – and the No. 69 pick in the 2024 MLB Draft to the Chicago White Sox for Santos, who was a rookie in 2023. That might have seemed like a steep price to some at first glance, but looking closer at Santos reveals an arm that could make for quite the combination with Seattle’s other top relievers.

The hard-throwing, 24-year-old right-hander threw 66 1/3 innings for the White Sox last season to the tune of a 3.39 ERA, 1.296 WHIP and 2.56 FIP, with 66 strikeouts to 17 walks. What especially stands out is Santos was MLB’s best in limiting barrels, ranking in the 100th percentile in that category per Statcast, as well as 78th percentile in hard-hit rate and 87th percentile in groundball rate. He can reach up to 103 mph with his fastball, averaged 98.8 mph with the pitch, and allowed only two home runs in 2023.

Add Santos to Andrés Muñoz and Matt Brash at the back end of Seattle’s bullpen, and that’s a dominant trio for opposing batters to worry about.

The analytical types also really like Santos, who started his MLB career with the San Francisco Giants in 2021. His ZiPS projection has him at a 1.5 WAR for the 2024 season, which is the highest for any relief pitcher. That’s so high that it caught the attention of FanGraphs senior writer and ESPN contributor Dan Szymborski, the man behind ZiPS, who said Sunday on social media that he had to jump into Santos’ projection to make sure everything was correct.

Whether or not the 1.5 WAR is a legitimate projection, it’s clear the Mariners have made a very interesting addition to their bullpen.

Insider view on Seattle Mariners’ Santos

On Monday morning, Seattle Sports’ Brock and Salk caught up with play-by-play announcer Jason Benetti, who saw Santos up close last season in his job with the White Sox (Benetti has since taken the same role with the Detroit Tigers this offseason).

One pitch in particular jumps out about Santos, a two-seam fastball (sometimes referred to a sinker), which Benetti said was new in 2023.

“He’s this kind of untapped mineral,” Benetti said. “And you’re talking about all the metrics that suggest he’s going to have a breakout year, and I wouldn’t disagree. I mean, he just completed his first season with a two-seamer. So the White Sox pitching coach Ethan Katz had him in San Francisco very early on when Ethan was the assistant there, and then Gregory ends up with the White Sox and Ethan converted his four-seam fastball to a two-seamer, and it was all about spin efficiency.

“So for those that are uninitiated, spin efficiency is about how much of the spin you’re putting on the ball is actually contributing to movements. Ethan felt like, specifically, that the four-seamer just wasn’t doing as much, (and) that the sinker paired better with the slider, as we’ve seen in baseball lore. He had very good success with his sliders specifically off of the sinker.”

Benetti wonders if the Mariners, who have had a lot of recent success in molding relief pitchers into better versions of themselves, have something up their sleeve to try with Santos.

“There was some talk of maybe at some point adding a cutter, as well,” Benetti said. “I would be interested in what the Mariners are going to do with his pitch mix, if they’re going to add one (pitch). There was some discussion of that, but just a second year of having the sinker full-time I think is important to him as he toys with that and plays with it.”

Santos ended the 2023 season on the injured list with right elbow inflammation, but Benetti, who also calls FOX football games alongside Brock and Salk co-host Brock Huard, isn’t as concerned about that as he is with the Tigers’ hitters eventually having to face the Seattle Mariners’ bullpen.

“I don’t think (the Mariners) would have done the deal if they had concerns about that. I think that was just more fatigue than anything – fatigue-based,” Benetti said of Santos’ elbow inflammation. “… I wouldn’t have imagined they would have done the deal if there was concern that was emergent. But yeah, I mean, he throws really hard and that is always a thing. First of all, I’m glad the Tigers don’t see the Mariners until August. Like, get all that stuff in the bullpen out right. I was talking to Brock off the air, I don’t think there’s a bullpen with more stuff – capital ‘S’ stuff – in Major League Baseball.”

What does Benetti make of Santos’ sky-high 2024 projection?

“I’m not trying to demean the guy, but I didn’t watch last year and say, ‘That is the best reliever in baseball.’ And that’s because there are really, really, really good relief pitchers in baseball,” he replied. “But I’m not surprised based on the stuff and based on the upside, right? So if you calculate into that, bake into that the idea that he was just learning a two-seamer and you can manipulate that pitch mix, I do think the ceiling is pretty substantial. And the walk rate can come down. He wasn’t always great after getting ahead of hitters. Sometimes it kind of faded on him, the at-bat did. I just think with some more maturity of major league pitching time, there’s a fantastic arm in there. There really, really is.”

How good is Jorge Polanco?

With Benetti having been in the American League Central for a while, it made sense to get his read on the other big Mariners trade acquisition from last week: Jorge Polanco, a switch-hitting second baseman and one-time All-Star the M’s got from the Minnesota Twins.

Benetti said he’s always been impressed by Polanco’s performances against the White Sox.

“Jorge Polanco is one of those guys that when he makes contact – I have like five missiles in my mind to the pull side that got out of the ballpark within, I don’t know, a second off a White Sox pitcher over the past bunch of years,” he said. “I’ve always really liked him. The contact rate isn’t always what you’d want it to be, but as a switch-hitter and as a guy who has more pull-side power than you would expect. I think he’s a really nice piece.”

You can listen to the full Brock and Salk conversation with play-by-play announcer Jason Benetti in the podcast at this link or in the player near the top of this post.

More on the Seattle Mariners

• Morosi: Trading for Polanco was move Mariners ‘had to make’
• New DH Mitch Garver has interesting insight on Mariners’ pitchers, trade additions
• Salk: Did the Mariners ‘find a way’ to get better this offseason?
• Where does Mariners’ pitching depth stand now? See what Dipoto said
• Seattle Mariners’ Dipoto: Why longtime trade target Polanco ‘fits perfectly’

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