OAKLAND, Calif. — The Mariners simply couldn’t let Justus Sheffield continue the path that he was on with Class AAA Tacoma. It wasn’t working for their top pitching prospect and sending him to the mound every fifth day was becoming more hurtful than helpful to his development.

On Friday, multiple sources in the organization confirmed that Sheffield was being sent down to Class AA Arkansas following another awful outing for the Rainiers.

It’s a stunning move given Sheffield’s status as a prospect. Acquired from the Yankees as the centerpiece of the three players that the Mariners picked up in return for lefty James Paxton, Sheffield was expected to be a part of the Mariners’ starting rotation by midseason.

But after impressing manager Scott Servais and the Mariners with his potential during spring training, Sheffield has endured a miserable season with the Rainiers that has featured no fastball command, a lack of pitch efficiency, far too many walks and so many runs scored.

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On Thursday night in San Antonio, Sheffield never made it out of the second inning, giving up eight runs on eight hits, including three homers, a walk and three strikeouts. It dropped his record to 2-6 with a 6.87 ERA in 12 starts and one extended relief appearance for Tacoma. In 55 innings, he’s allowed 59 hits and 42 runs with a whopping 41 walks and 48 strikeouts.

In those 12 starts, he’s pitched past the sixth inning in just two. He’s failed to make it past the third inning in his last three starts, giving up 20 earned runs in seven innings for a 25.71 ERA with 11 walks and eight strikeouts.

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Sheffield made one appearance with the Mariners, piggybacking off Yusei Kikuchi’s one-inning start versus the Rangers on April 26. He pitched three innings, allowing two runs on two hits with four walks and three strikeouts.

Per multiple sources, Sheffield’s struggles stem from mechanical inconsistencies and an inability to slow things down when he gets runners on base.

Because of his size — 6-foot, 200 pounds — he’s got a maximum-effort delivery that’s difficult to repeat consistently over a start. There is a belief that Sheffield never really developed a consistent delivery because of a rushed trajectory because of being left-handed and having a mid-90s fastball and being traded multiple times.

The work that he’s done to try and correct things is positive in between starts. But his ultracompetitive nature and his abundance of emotion leads him to forget the work done during the week, reverting back to old habits.

An opposing scout labeled it as, “overaggressive and out of sync with a natural default to amp up rather than pull back.”

But most scouts are quick to point out that Sheffield is only 23. His stuff and athleticism are still above average. But there’s a belief that his projection isn’t for 2019 and possibly not as a starting pitcher but as a reliever. That would be a setback of sorts for the Mariners, who are counting on Sheffield to be a part of their future rotation. They targeted him as a key prospect when the decision was made to trade veteran players in hopes of adding young talent to the system.