JUPITER, Fla. -- Andrew Miller has lost control, and neither he nor the Cardinals know why.

The two-time All-Star and 2016 ALCS MVP with Cleveland will be sidelined for an undetermined amount of time as he tries to regain the feel for his pitches.

"The sensation I have throwing a baseball now just isn't consistent with what it is when I know I'm good," Miller said.

Miller struggles to describe the feeling he experiences when releasing the ball. It's not pain. There's no burning or tingling. It simply doesn't feel right.

Scheduled to pitch in relief Monday, Miller cut short his warm-up after several tosses badly missed his target.

"I wasn't worried about hurting myself," Miller said. "The stuff that's been bothering me and I've been trying to work through, it just wasn't getting better."

That outing followed a wild Grapefruit League debut Friday during which Miller allowed one run in one inning while hitting two batters, including Tim Tebow.

Miller said the issue dates to last season, his first with St. Louis, and contributed to his disappointing 5-6 record and 4.45 ERA. He's entering the final year of a two-year free-agent contract worth $25 million, with a club option for 2021.

"There's not an incident or moment when something happened and you can really point to something," Miller said. "Whatever it is, it's been gradual to get there."

Miller said his arm and shoulder feel fine and aren't experiencing weakness. He spent Tuesday undergoing tests.

"In some ways it's good that there's no clear-cut pain," Miller said. "In other ways it's frustrating because, what is it?"

Entering spring training, Miller was among a handful of relievers who could have emerged as the Cardinals' closer, but he seemed destined to reprise his setup role as a lefty capable of throwing multiple innings.

Miller is weary of trying to pitch through the issue without having any idea of the cause.

"Last year was a challenge," Miller said. "Even before that, you keep waiting, you keep putting the work in and hoping you kind of pull through the fog and you can do what you expect from yourself. It's just been an incredible grind."

The injury news is significantly better for Kwang Hyun Kim. Groin tightness prompted the offseason signee from Korea to skip Monday's scheduled start and throw a bullpen instead.

Kim emerged healthy from that session and is now scheduled to start one of the Cardinals' games on Thursday.

Manager Mike Shildt said the Cardinals aren't overly concerned about the split fingernail that forced Genesis Cabrera, Kim's replacement starter Monday, from that game.