"I've been struggling up here for a while," Miller said in the Arizona clubhouse Thursday. "I'm surprised I stayed up here this long."
Miller has been a major disappointment after the Diamondbacks got him in an offseason trade with Atlanta for outfielder Ender Inciarte, 2015 No. 1 overall draft pick Dansby Swanson and top pitching prospect Aaron Blair.
"We just have to find a way to get him back to pitching the way he can," manager Chip Hale said.
The right-hander is 2-9 with a 7.14 ERA. He was on the disabled list with a finger injury and worked in the minors to find his form. After some progress in is return, Miller had another rough night last week in a 13-6 home loss to San Diego. After blanking the Padres for three innings, he gave up five runs in the fourth when Arizona had a 4-0 lead.
"His stuff was good at times, good enough to win here," Hale said. "He just couldn't put it together, so he'll go down there, work on it and pitch his way back to the big leagues."
Immediately after giving up so much to get Miller, the Diamondbacks penciled him in as the No. 2 starter behind Zack Greinke. But the team stumbled out of the gate and finds itself in last place in the NL West (38-52), 19 games behind first-place San Francisco.
"I told him we're not going to be the team that we envisioned without him being who Shelby Miller can be," Hale said. "That was my message."
Hale said left-hander Edwin Escobar would be called up from Reno to take Miller's roster spot. Escobar will work out of the bullpen, Hale said, and Zack Godley will be recalled from Reno to start Tuesday against Toronto.
Miller said he pored over videos of his pitching during the break and said he saw some differences between this year and last.
"The biggest thing is to get back to maybe a little bit simpler and not so much forcing things and trying to make a perfect pitch," he said. "I think that's when mechanics come easier and everything kind of flows and feels free. Hopefully I can get to that point and get back up here."
Hale said Miller's mechanics seem the same as they were last season.
"The mental side of it is just really hard to deal with," Hale said. "When you don't have success, it festers and it gets worse."
Hale called it "a disappointing move for us to make as coaches."
"We feel like we're here to make them better, make them better ballplayers and pitchers," he said, "and we've dropped the ball with him."
Miller said he had no idea how long he'd be gone.
"It could be the rest of the year, it could be a couple of weeks, who knows," he said.
Hale said Miller would have to earn his way back, like any other player.
"It's a performance-based game," Hale said.
The Diamondbacks also don't have Greinke, who they signed to a six-year, $206.5 million contract in the offseason. He's on the 15-day disabled list with a strained oblique.
Arizona is home against the Los Angeles Dodgers on Friday night.