Derrick Goold with the news:

Shelby Miller moving to the bullpen tonight until ... Well, not a guarantee for Wednesday. Matheny says it's for rest. #STLCards #cardinals — Derrick Goold (@dgoold) July 18, 2014

Matheny said that a four-man rotation is now in play for the next turn. Could stick with it for awhile, Matheny said. #STLCards #cardinals — Derrick Goold (@dgoold) July 18, 2014

From @dgoold: I asked Matheny about sending Shelby to the minors to get work instead of going to the bullpn, and he said this is about rest. — Tim McKernan (@tmckernan) July 18, 2014

It's hard to know how long this will last or what it means. With off-days July 21, 24, and 28, the Cardinals could easily pitch just Wainwright, Lynn, Martinez and Kelly until August. Miller suffered from mechanical issues and back pain in the last several weeks, so the "rest" reasoning given by Matheny certainly makes some sense.

But Miller has struggled all season, and it's possible that this is a bullpen trial for him. Michael Wacha will have another exam soon, and if all goes well, he could be on his way be back before too long. When Wacha returns, might Miller be the odd-man out indefinitely?

It's also easy to imagine that this could be related to a potential trade, but there's nothing that could suggest it other than the calendar.

Speaking of the calendar, Shelby had a 2.92 ERA and a 3.07 FIP at the All Star Break last season. A lot can happen in a year.