When Seattle selected JP Sears 333rd overall in this year’s draft, they knew they were getting a pitcher with a propensity for punch outs. In his junior season at The Citadel, the 21-year-old left-hander fanned 142 batters — the most in Division I baseball — in 95.1 innings. What they couldn’t possibly have known was that his strikeout rate would rise once he got to pro ball.

In 17 relief appearances between short-season Everett and Low-A Clinton, the 11th-round pick struck out — drum roll, please — a staggering 51 batters in 27.2 innings. He also allowed just 13 hits and two earned runs.

You can’t hit what you can’t see, and according to Clinton Lumber Kings pitching coach Doug Mathis, that’s the secret to the southpaw’s success.

“I had him here for five or six weeks, and he basically used one pitch,” Mathis told me on Monday. “If he goes out and throws two innings and 30 pitches, probably 27 of them are fastballs — and hitters can’t hit it. It’s not elite velocity by any means. He can touch 93 — nowadays that’s average in the big leagues — but guys don’t see it well or react to it well. It’s an invisiball.

“Even their takes are bad. When they do swing the bat, it’s as though he’s throwing 95-100, and not the 90 the radar gun is showing. You look at the readings and go, ‘How is he doing it?’ The ball is just different coming out of his hand. It kind of has an extra gear.”

Sears has an above-average spin rate on his four-seamer, an average of 2,350 during his time in the Midwest League. A deceptive delivery makes it play up even more. Mathis said Sears doesn’t throw across his body, but rather that “he kind of gets his arm down long, and then he gets it up kind of slow; from there it’s smooth forward.”

Bumpy command in his first outing under Mathis’s watchful eye — three walks in two innings — led to some sage advice.

“I told him that he didn’t have to be so fine, especially at this level,” explained Mathis. “I told him to watch some video and see the reaction of the hitters against him — watch their swings, watch their takes. Basically, he was trying to pitch too much when he didn’t have to. The way his fastball plays, what he needed to do was just attack.”

Going forward, the Sumter, South Carolina, native will need to add nuance to his game. As Mathis acknowledged, it will be necessary for him to command the baseball better as he climbs the minor-league ladder. Ditto with mixing in his other pitches. Along with his signature offering, Sears has a “decent changeup” and a “breaking ball he can spin OK.”

The keep-it-simple advice was predicated by the situation. Like most organizations, the Mariners are mostly hands off in a player’s nascent months of pro ball — plus, Sears was pitching out of the bullpen to keep his innings under control. Mathis expects the 5-foot-11 lefty to get an opportunity to start next season, but at the same time, he comped him to a reliever.

“He kind of reminds me of Jeff Beliveau,” observed Mathis. “Not so much the delivery, but their size is similar, their velocity is similar, the way the ball comes out of their hand… again, hitters just don’t see the ball well. I think that can translate [to the big leagues]. There are obviously things he needs to work on, but not a lot of pitchers have a fastball that plays like that.”

It’s hard to argue. When you strike out 16.6 batters per nine innings in your first foray against professional hitters — using almost exclusively one pitch — your fastball definitely plays. Or, as one might put it, your invisiball definitely plays.