TACOMA — It was a short line in a small clubhouse. Justus Sheffield shuffled through the Chipotle-catered options laid out on a hallway table, filling the top layer of his aluminum bowl with a scoop of black beans, lettuce and shredded cheese.

This was Monday afternoon, three days after Sheffield’s latest puzzling start for the Class AAA Tacoma Rainiers, and six weeks after his one cameo appearance out of the Seattle Mariners bullpen.

On April 26, Sheffield threw 75 pitches against the Texas Rangers at T-Mobile Park. He allowed two runs, both coming on a Nomar Mazara home run, with three strikeouts and four walks in three innings.

That wasn’t his first taste of the major leagues — he made three relief appearances for the New York Yankees last year — and, surely, it won’t be his last.

And yet, as he slogs through the worst stretch of his professional career, the Mariners’ top pitching prospect doesn’t appear to be anywhere close to a return to the big leagues. From Cheney Stadium, Seattle is so tantalizingly close and yet so far away for a pitcher struggling to throw strikes.

And that, of course, is where the 23-year-old left-hander wants to be, back on the mound at T-Mobile Park, back in a big-league clubhouse. And that, maybe, can at least partially explain some of Sheffield’s struggles since his return to Tacoma — the frustration of a demotion and the distraction of the dangling carrot that is the big leagues affecting his focus and production.

“Yeah,” Sheffield said during an interview in the Tacoma clubhouse. “Being there (in Seattle) and seeing it, obviously that’s the final goal. It’s taken a little longer than I wanted so far, but I really can’t be thinking on that. I just gotta think day to day (about) getting better and taking the positives out of my negative outings.”

There have been a lot of those negative outings lately.

In seven Triple-A starts since May 1, he has a 6.69 earned-run average, with a 31-to-26 strikeout-to-walk ratio in 35 innings.

Overall, he has a 5.74 ERA in 11 starts in the hitter-friendly Pacific Coast League this season, and his 40 walks (in 531⁄3 innings) are second-most in the league. According to FanGraphs, he’s striking out 18.4% of the batters he faces, a career low, and walking 16% of batters, a career high.

In his most recent start, in Tacoma on Friday night against Nashville, he was tagged with five runs in 22⁄3 innings. He was nearly untouchable through his first two hitless innings, striking out three and walking one. But he couldn’t get out of a third inning in which he walked four and allowed two singles.

“It’s frustrating,” he said. “It’s very frustrating, because I know what I’m capable of, and I know where I want to be, and I know how I’ve pitched in the past. It’s been a long time since I’ve been in a little rut, but, you know, that’s the game. And you’ve got to keep clawing and fighting and scratching to get back to pitching.”

Sheffield says he hasn’t lost his fastball, hasn’t lost the feel for his above-average slider, and it’s clear he hasn’t lost confidence.

He did acknowledge that he has lost focus, and his way, in recent starts.

“The situation I’m in right now — obviously, I’m trying to get back to the big leagues, but that can’t be on my mind,” he said. “It’ll happen when it’ll happen. The biggest thing for me right now is just get better, personally.”

One talent evaluator who has scouted Sheffield on multiple occasions told The Times that Sheffield’s delivery has been “out of sync” — that the left-hander has a tendency to rush forward with his windup.

“There are lots to get through with (Sheffield),” the scout said, “but none of it is unfixable.”

Sheffield said he’s been watching video of his past outings to find some answers. There are “minor” things he’s found, but the overarching issues, he said, aren’t physical.

“It’s not the stuff, which is crazy. My arm feels great. My stuff’s been great,” he said. “It’s just throwing it over the zone, trusting the stuff and letting the hitters get themselves out. You can’t be a starting pitcher in any league going three innings. I’ve just got to get back to the basics, and that’s pounding the zone, getting first-pitch strikes and getting ahead.”

Andy McKay, the Mariners’ director of player development, said he was encouraged by Sheffield’s strong start against Fresno on May 22 (one run and one walk in seven innings), and encouraged by the 23-year-old’s work ethic to turn things around.

“My confidence in Justus has not wavered a bit, because I know his confidence has not wavered a bit,” McKay said. “That’s the mark of someone who’s going to be great, and that’s what I respect about the way Justus has gone about this.”

McKay was quick to point out that it wasn’t that long ago that another prized left-hander in the Mariners’ system, James Paxton, had several up-and-down seasons in Triple-A before eventually sticking in the majors. Paxton, of course, was traded to the Yankees last winter to kick-start the Mariners’ rebuilding plan, and the centerpiece of the package coming back from New York was a promising young left-hander who is trying to work through his own issues in Tacoma.

Sheffield is sure he will get things turned around.

“I’m not stressin’ at all,” Sheffield said. “Like I said, I definitely would rather be in a better position at this point in the year, but it is what it is. God wants me to be here, and he wants me to work on things. I’ll put some trust in Him and put some trust in myself.”