Rocker, the Rangers’ 2022 first-round pick, didn’t fold in the first inning after the first two batters he ever faced collected base hits.

SEATTLE — The Trackman technology at T Mobile Park labeled the nastiest pitch thrown Thursday night as a slider, and so did just about everybody else during and after the Texas Rangers’ 5-4 victory.

Even the pitcher who threw it, Kumar Rocker, went with the flow when asked about the slider … even though he considers it a curveball.

Someone alert the tech nerds.

Whatever it is, the pitch produced 13 swings and misses from Seattle Mariners hitters, some of whom were made to look foolish by the 24-year-old making his MLB debut.

Rocker was, at times, spectacular in the first big-league start of his career, as his seven strikeouts in four innings indicate. His breaking ball certainly was, and his two fastballs were pretty good, too.

The Rangers, though, went back to the team hotel more impressed by the way Rocker handled himself in the biggest moment of his baseball career, and he showed his poise from the start.

After three pitches, the Mariners had two hits. After five pitches, they had runners and second and third with no outs. At the end of the first inning, though, they had no runs.

“What stands out is his composure, the poise that he showed out there,” manager Bruce Bochy said. “That first inning, trust me, that’s as nervous as you can be when you make your first major-league start. He didn’t let it get away from him, made pitches. He just looked so focused out there, just the presence of how he carried himself out there.”

Rocker admitted to some nerves and extra adrenaline, more than he realized were coursing through him. The singles by Victor Robles on Rocker’s first MLB pitch and Julio Rodriguez weren’t hit hard, both with exit velocities below 80 mph, but they found holes. They also shifted Rocker into a different gear.

He came back with his first career strikeout on a 97-mph heater that froze catcher Cal Raleigh. Former All-Star outfielder Randy Arozarena followed and went down swinging. Luke Raley hit a scorcher to right field, but Travis Jankowski corraled it for the final out.

“It was time to play baseball early,” Rocker said. “There’s no easing into. Got right to it.”

He picked up two more strikeouts in the second, including one of veteran Jorge Polanco. He fanned on three sliders, looking completely fooled on the first two.

Rocker pitched around two walks in the third inning to keep the Mariners scoreless, but Justin Turner hit a grooved fastball over the center-field fence in the fourth. Rocker added to more strikeouts in the frame, but at 74 pitches, the Rangers didn’t send him back out for the fifth.

The performance — three hits, one run, two walks and seven strikeouts — left most impressed. Rocker, however, saw things that must be corrected for his second MLB start next week at Globe Life Field.

His command wasn’t good enough with the fastball and the breaking ball. That led to another issue, not putting away hitters quickly enough when ahead 0-2 in the count. It didn’t happen often, but it helped contribute to his pitches piling up.

But he also recognized that the moment might have contributed to what problems he did encounter.

“I feel like the whole game I was trying to find my rhythm, and that’s just part of the moment,” said Rocker, the third overall pick in 2022. “That’s part of it being a new experience. You can’t really get frustrated from it but learn from it.”

Of course, the Rangers loved the stuff. Rocker flourished with his two fastballs and the breaking ball, but his poise won them over.

“He has the swing-and-miss stuff to get out of jams,” second baseman Marcus Semien said. “Obviously, he was pretty amped up, and two hits in a row, it’s time to go. He locked it in.”

Jeff Wilson, jwilson@alldlls.co