Discipline Makes All The Difference For Mariners’ Jonatan Clase

As the fastest prospect in the Mariners’ system, Jonatan Clase is most known for his blazing speed.

But ultimately, it was the 20-year-old outfielder’s red-hot bat that sent him racing up to Double-A this spring.

Coming off a strong 2022 in Low-A Modesto, Clase stormed out of the gates this season to slash .333/.453/.701 with seven home runs and 17 stolen bases in 21 games at High-A Everett.

The scorching start earned the Dominican switch-hitter a May 3 promotion to Double-A Arkansas, making him one of the rare Mariners position prospects to reach that level before age 21.

“It was very impressive,” Everett hitting coach Mike Fransoso said.

At the heart of Clase’s breakout spring was his improved plate discipline.

The muscular 5-foot-9 slugger posted a 17% walk rate in Everett, while showing considerable growth in laying off breaking balls out of the zone. His strikeout rate was still high at 26%, but Fransoso noted the majority of his punchouts came looking.

“The at-bats he (had) were very professional and very mature from where they were even last year—let alone when he first came over to the States,” said Fransoso, who also was Clase’s hitting coach the previous two seasons.

“He was swinging at the right pitches (and) he wasn’t chasing a lot.”

Another major factor has been Clase’s rapid development as a switch-hitter. 

Clase exclusively hit lefthanded from about age 14 to 19. But when he was younger, he had experience batting righthanded.

That old skill reemerged in the cage one day back in 2021, when Fransoso spotted Clase taking righthanded cuts off a machine for fun. As Clase smashed breaking ball after breaking ball, Fransoso took out his phone and recorded it on video.

Club officials liked what they saw and gave Clase the green light to start training from both sides. By the start of the 2022 season, he was switch-hitting in games.

“It’s a crazy story,” Fransoso said. “It happened quick.”

Clase still did the bulk of his damage from the left side last year, posting an .866 OPS batting lefthanded and a .750 OPS batting right.

But in Everett this spring, Clase put up big numbers on both sides. His lefthanded OPS improved to 1.071 and his righthanded OPS skyrocketed to 1.337.

“The progression on the right side really took off,” Fransoso said.

Heading into the season, Clase changed the hand setup on his righthanded swing. He also uses two different bats now, opting for one with a puck knob when batting righthanded.

“Last year, I think he was trying to just replicate his lefthanded swing on the right side,” Fransoso said. “. . . Now he’s really starting to feel the difference between the (two).” 

Clase showed power on both sides during his short stint in Everett, blasting four lefthanded homers and three righthanded shots.

It continued a recent home run surge for the 2018 international signee, who has packed on significant strength since signing. 

In his 2019 pro debut, Clase homered just twice in 63 Dominican Summer League games. But after the canceled 2020 season and an injury-plagued 2021, he hit 13 homers in Modesto last year.

“He’s got such a quick, pretty swing that when he squares it up and hits it at the right angle, it’s gonna go,” Fransoso said.

Clase now looks to continue his success in Double-A, where he already has a walk-off single, four extra-base hits and nine stolen bases through 10 games. 

“He’s really starting to understand who he is as a player (and) as a hitter,” Fransoso said. “He’s just matured a ton. And he’s still young . . . He’s still learning.”



— Top Mariners pitching prospect Bryce Miller was off to a spectacular start to his major league career. The 24-year-old righthander allowed only one run and seven hits in 19 innings, while recording 18 strikeouts and only one walk.

Miller took a perfect game into the sixth inning of his May 2 debut against the Athletics, retiring the first 16 batters he faced. He then tossed six scoreless innings against the Astros and seven shutout frames against the Tigers.

Miller produced 15 strikeouts with his electric four-seam fastball, which has averaged 95.5 mph and generated some of the best vertical movement metrics in the majors.

— Righthanded reliever Juan Then also made his major league debut earlier in May. The 23-year-old Dominican pitched 4.1 scoreless innings across four relief appearances for the Mariners, retiring 13 of 17 batters faced.

— Double-A Arkansas righthander Bryan Woo continued to show why he’s one of the Mariners’ top pitching prospects. The 23-year-old had a 1.93 ERA through 28 innings in the Texas League this season, with 39 strikeouts and just six walks. He led the league with a 0.71 WHIP and ranked second with a 37.1% strikeout rate. Woo is still less than a year removed from his pro debut, which came last June after he recovered from Tommy John surgery.

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