"We're going to give him a day to grab him time in the training room and work on that," manager Mike Matheny said. "We're not too concerned. We're just going to give him a little more time. It's more preventative right now."
Miller guessed that the soreness was the result of going full effort during his live batting practice, which he threw on Saturday. That effort level was one that he hadn't reached since last year. Miller pointed out a key distinction, too: The soreness cropped up not while he was throwing, but afterward.
The issue was not deemed serious enough to warrant Miller scheduling an MRI. On Tuesday, he took an anti-inflammatory pill, which he said helped significantly reduce the soreness.
"I'm just taking it easy," Miller said. "The biggest thing is just they're making sure there is nothing wrong with me. They just want to make sure it's nothing. That's it. It's nothing serious at all."
Miller projected that he could resume throwing on flat ground as early as Thursday. Matheny did not speculate on when Miller will return to the mound.
The club's initial plan was to have the righty pitch in one of the team's first Grapefruit League games. That season begins on Saturday. But because the Grapefruit League schedule is longer this year, Miller will have plenty of time to get his arm ready for the start of the season, even if he gets a few days behind schedule.
"We planned on having Shelby out there pretty early, but we're going to go slow," Matheny said. "We have all kinds of time. This isn't a major issue as much as trying to get ahead of something that is more preventative."
Miller, along with Trevor Rosenthal and Joe Kelly, is competing for the final spot in the rotation. Rosenthal will be the first of the three to throw in a game; Matheny named him the starter for Saturday's Grapefruit League opener against the Marlins.
Miller has been widely considered to be the Cardinals' top pitching prospect since he was taken by the organization in the first round of the 2009 First-Year Player Draft. He made his Major League debut in 2012 and was recently ranked by MLB.com as the organization's second-best prospect heading into the 2013 season.