When Eric Longehagen and Kiley McDaniel blurbed Sam Delaplane last March, they called the Seattle Mariners pitching prospect “an interesting sleeper.” Pointing to his eye-popping strikeout numbers in Low-A, they went on to suggest that Delaplane — unranked despite the platitudes — “might get pushed quickly.”

Delaplane proceeded to prove our scouting duo correct. Following 21 relief outings in Hi-A Modesto, the righty ascended to Arkansas, where he flat out shoved against Texas League hitters. In 37 Double-A innings, Delaplane fanned 58 while allowing just 13 hits. His ERA was a microscopic 0.49.

Flash back six years. In order to compete collegiately, the San Jose native had to travel 2,400 miles — and not to a baseball hotbed. The lone offer Delaplane received coming out of high school was from Eastern Michigan University; the low-profile program was coming off of consecutive losing records in the Mid-American Conference.

Delaplane spent four years at Eastern, earning a degree in marketing. Sold mostly on the promise of his strong senior season — a 3.27 ERA and first-team All-MAC honors — Seattle selected Delaplane in the 23rd round of the 2017 draft.

His most-lethal weapon had yet to evolve and blossom. It wasn’t until after Delaplane got into pro ball that he “flipped the switch” and turned a hook into what Longenhagen described as a “power, Brad Lidge-style slider with late, downward movement.”

Defining Delaplane’s best offering is a matter of semantics.

“My primary pitch used to be a curveball,” explained the erstwhile Eagle. “And personally, I still consider it a curveball. That’s how I think of it. Technically it’s a slider, but it was a curveball grip when I first started throwing it, at 14, and that’s never changed. I feel like I’m coming over the top, and then turning a doorknob. If I think slider, it’s going to get more sweepy, right-left, which isn’t what I want. I’m thinking up-down, like a 12-6 curveball.”

The velocity is akin to that of a slider. Deplane divulged that on his best days the pitch will be between 85-88 mph, and on lesser days it will be 83-85. His four-seamer, which features late life, ranges anywhere from 93 to 96.

Just how good is his signature pitch? Per Baseball Savant, the 25-year-old (as of last Friday) “had the second highest swing-and-miss rates for a slider of anyone in baseball” last year. A good many of them came with two strikes on the batter. When I asked about his strikeout rate, Delaplane sounded almost sheepish in his response: “I think it was 15.7.” (It was.) For good measure, he proceeded to punch out 15 Arizona Fall League hitters in eight innings of equally-overpowering fashion.

He professes to not hunting strikeouts. But only to a point.

“I’m just trying to get ahead, and they just kind of happen when you get ahead,” reasoned Delaplane, who now ranks 11th on our Mariners Top Prospects list. “If I’m locating and doing what I need to do, it… I mean, I’m honestly not out there looking for strikeouts. I’m trying to get guys out as quickly and efficiently as possible. That said, if I get ahead 0-2, I do try to put them away.”

Cal Raleigh has seen that first hand. The No. 9 prospect on our Mariners list caught Delaplane in both Modesto and Arkansas. Along with affirming the righty’s raw stuff, Raleigh lauded his ability to locate, adding that Delaplane is locked in on the mound and works with a good tempo.

Unsurprisingly, Delaplane has a decent amount of adrenaline coursing through his veins when he toes the bump. Being a power reliever, that comes with the territory.

“Yeah, there’s a little bit of it,” Delaplane admitted. “Sometimes that can be to my detriment, but I’d say it benefits me more often than not. Sometimes I’ll get a little sporadic out there, but usually that’s with the fastball, and not the slider.”

As Raleigh attested, Delaplane doesn’t have issues with the strike zone. And not only did he walk just three batters per nine innings last year, his control got crisper as the campaign progressed. Over his last 18 outings, Delaplane issued a paltry four free passes in 26-and-a-third frames. Along the way, he logged 36 strikeouts.

Longenhagen didn’t pull punches in the youngster’s 2020 scouting summary. One year after calling him “an interesting sleeper,” our resident prospect guru proffered that “Delaplane will be a reliever of rare quality.”

Again, this is a pitcher who drew scant interest out of high school, and was a late-round senior sign out of a low-profile college. Simply reaching the big league would qualify as rare. Due largely to the quality of his slider, Deplane is seemingly on his way to doing just that.