Miller is battling lower-back soreness and will be receiving treatment during the week, Scott Lauber of The Philadelphia Inquirer reports.
It's not quite clear when this issue popped up for Miller, but the team is understandably proceeding with caution to ensure the injury doesn't get worse. The 21-year-old received an invitation to big-league camp and has an outside chance to make the Opening Day roster, but this injury won't help his chances. Miller can be considered day-to-day until the team has another update to share.