SEATTLE -- While Gerrit Cole powered his way to an Astros record for most strikeouts in a season, Mariners rookie Justin Dunn took a much smaller step Tuesday night at T-Mobile Park. But for Dunn, a smooth two-scoreless-inning start in a 3-0 loss stood as a promising sign as he begins his Major League career.

Dunn, ranked by MLB Pipeline as the Mariners’ No. 5 prospect, is one of several youngsters getting their feet wet in September for a Seattle club looking toward the future. The 23-year-old’s initial two starts were shaky, as he walked eight of the 16 batters he faced, but Dunn allowed just an infield single to José Altuve in two quick frames against the American League West champion Astros.

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“I felt pretty comfortable out there, like I’m getting better every time out,” Dunn said. “It’s still the big leagues and I’m still getting used to it, but my heart rate is starting to slow down a little bit and the stadium’s not looking as big anymore.”

Dunn didn’t issue any walks in his 27-pitch outing, while striking out Kyle Tucker, and didn’t allow a ball out of the infield as he lowered his ERA to 3.86.

“The highlight for us tonight, Justin Dunn threw the ball great,” Mariners manager Scott Servais said. “He definitely took a big step in the right direction against a very good ballclub. His tempo was great, his ball had life to it, a lot of good breaking balls. A big step for him as he continues to move down the road in his career. I like what I saw there a lot.”

The Mariners are limiting Dunn’s innings after he pitched a full season at Double-A Arkansas, using him for abbreviated starts and then turning things over to veteran lefty Tommy Milone. Dunn will get one more short start in Sunday’s season finale against the A’s and then begin offseason preparation for an attempt to crack Seattle’s rotation next spring.

“He’s got a lot of talent,” Servais said. “He’s really smart, and he knows he just needs the reps and innings and that’s what we’re going to give him. I’m sure that’s what he looked like the majority of the year in Double-A. We just hadn’t seen it yet. Now we have, and he can build upon that one.”

The Mariners managed just two hits off Cole in his seven innings as the hard-throwing right-hander racked up 14 strikeouts, giving him 316 on the season to break the franchise record J.R. Richard set in 1979. The last AL pitcher with more strikeouts in a season was Nolan Ryan with 341 in 1977.

Cole had 50 strikeouts in 28 innings over four starts against Seattle this season, going 4-0 with a 1.29 ERA. He’s 19-5 with a 2.52 ERA overall.

“What are you going to say? He’s basically dominating the league right now, and we’ve seen him a few too many times this year,” said Servais. “He had it going on tonight, all four pitches working, locating them, threw a ton of changeups. It looked like he was working on that pitch for a little while. He’s having an outstanding season, and he shut us down. We’re not the only team he’s shut down this year or will shut down the remainder of their season.”

The loss dropped the Mariners to 1-17 on the season against the Astros, with one game remaining Wednesday. The Tigers went 1-18 against the Indians this year to become the first team since MLB broke the leagues into divisions in 1969 to lose 18 games against one opponent. Seattle will look to avoid becoming the second, though it’ll be another tough challenge with Zack Greinke on the mound for the Astros, who’ve now equaled the franchise-record 103 wins they totaled last year.