DENVER — Pete Woodworth, the Mariners’ pitching coach, loves to hear the grunt.

When Yusei Kikuchi is in total command of his arsenal, when his emotions are fully charged, when he’s attacking hitters with such aggressiveness that he emits sounds like a hard-serving tennis player, Woodworth said he knows he’s seeing an All-Star pitcher at the height of his game.

Kikuchi showed that sporadically in the past, but his inconsistency kept him from truly emerging as the ace pitcher that his ever-improving stuff indicated was lurking within. The key this year, Woodworth said, was to figure out, “How can that monster come out more often?”

To the Mariners’ ongoing delight, the monster has reared its head virtually every start. That’s why Kikuchi, after a brief COVID-19 scare over the weekend that was erased by a series of negative tests, is on the American League All-Star team for Tuesday night’s game at Coors Field.

Kikuchi on Monday night was a surprising late scratch for the game, but his turnaround has been remarkable. He struggled to adjust to MLB after coming to the Mariners as a much-hyped free agent in 2019. The stuff that had made him an All-Star in Japan wasn’t translating to his new league, and it was hard not to wonder if the Mariners had miscalculated in giving Kikuchi a minimum of $58 million over four years.

After a 6-11 rookie season with a 5.46 earned-run average, made even more trying by the death of his father, Kikuchi decided it was time for major changes.

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Instead of listening to all the voices in his ear, as he had tended to do, Kikuchi decided to hone it down to one consistent message. And immediately after the season, with the Mariners’ blessing and coordination, he approached the state-of-the-art analytic minds at Driveline Baseball in Kent for a detailed assessment of his throwing, complete with biomechanical data to back it up.

That, in turn, informed a series of tweaks to his mechanics that resulted in a significant upturn in velocity. And a much more efficient throwing motion has allowed him to sustain it.

“That was definitely huge for me,” Kikuchi said Monday via interpreter Kevin Ando at the AL media availability session. “Just seeing all the numbers to back up all the fact was very, very satisfying for me.”

The improved stuff Kikuchi flashed in 2020 didn’t result immediately in success on the mound. But his 5.17 ERA hid the fact that a true breakthrough was brewing. Kikuchi said he believes last year’s delayed 60-game season was impossible to accurately assess.

“I think the biggest thing about last year is I only had nine starts for the entire year, and I feel like I didn’t have enough time to completely solidify my new mechanics,” he said. “And so the biggest goal this year was to do that, to solidify my mechanics, and I think I’m able to be doing that currently.”

Bill Hezel, Driveline’s director of pitching, worked with Kikuchi for about two weeks after the initial weeklong assessment, and following Kikuchi’s two-week trip to Japan to decompress from the 2019 season. He said that Kikuchi’s deep abiding interest in analytics made him a perfect student.

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“He had a base understanding of pitch movement and things like that,” Hezel said. “So that just made the conversation very easy. We didn’t have to spend a ton of time going over a lot of that stuff. We spent more time actually just working on stuff.”

Hezel said the ultimate goal was to get more efficiency and ride back on Kikuchi’s fastball, and a better separation between his fastball and changeup. Without getting too technical on the mechanics, he worked most specifically on the fact that during delivery, Kikuchi’s lower half was too closed off when he got to his front foot strike, and he had an excessive trunk tilt.

“If you picture what he would look like when he gets the foot strike, he was really leaning back, toward third base,” Hezel said. “It was really excessive compared to where he was in 2017 and ’18.

“What ends up happening when that lower half gets really, really closed off, it can just can throw off the timing of events and inhibit the transfer of energy, kind of from the ground up.”

Pitching is such an intricate process that small changes can lead to major improvement, and that was the case with Kikuchi. He absorbed the lessons and drills from Driveline and took them to Arizona for a full offseason of training.

“The number one thing I took out of it was, and what they really emphasized, was on my take-back and my arm slot,” Kikuchi said. “And from there, I think my velocity took off.”

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This season, that improved stuff was married with Kikuchi’s more demonstrative attitude to forge one of the most dominating lefties in the league. Kikuchi’s average fastball velocity is nearly 96 mph, up significantly from the 92.5 of his rookie season.

“The very first night this year, vs. the Giants, we saw him get to that next gear, that next level of competing and aggression,” Woodworth said. “You know, he’s grunting, he’s showing emotion. And it’s been slowly and exponentially getting better every start.”

Said Kikuchi: “I think all that aggression and mentality has naturally kind of came out. Because, you know, I am comfortable with my mechanics, and I trust that everything is right. So I think it just comes out naturally.”

The Mariners face a flashpoint on Kikuchi’s unique contract after the season. They can exercise a four-year, $66 million option to keep him in the organization through 2025. But if the Mariners decline the option, Kikuchi can either exercise a one-year, $13 million contract for 2022 or opt to immediately become a free agent.

What once seemed to be a difficult decision now is turning into something of a no-brainer, barring a total collapse or injury by Kikuchi in the second half. At $16.5 million a year, the 30-year-old Kikuchi would be a bargain compared with the cost of replacing him on the free-agent market.

“He’s made a very compelling case for how he fits here for the long term, and that’s why we signed that contract,” Mariners general manager Jerry Dipoto said last week. “We wanted him to be here for the long term. I’m certain that he likes pitching here. He likes Seattle, I believe he likes the organization and his teammates, and you can see that he feels at home here.

“Those are all very positive things. And now I look back on the contract and the way we wrote it, and I’m very glad we did. It gives Yusei and the club great options either way.”

I suspect we’ll see Kikuchi grunting at T-Mobile Park for years to come.