I can't say the St. Louis Cardinals won't trade Shelby Miller, as rumored recently, because they sold incredibly low on a former top prospect just last year, when they dealt Colby Rasmus for Octavio Dotel and Edwin Jackson. That one worked out, though I can't quite give John Mozeliak credit for the way it did. But unless their potential trading partners aren't phased by Miller's rough introduction to the Pacific Coast League--he's walked 43 and allowed 17 home runs in 77 innings with Memphis--I can't imagine the Cardinals will be getting top dollar for a player who's been their top prospect for three years now.
Unless he's the linchpin to a major deal--for Zack Greinke, say, as the precursor to a Matt Holliday-style contract extension--the Cardinals would be trading years of value for a few months of a player near the front of the rotation. Miller's a step away from the major leagues with a strikeout rate over 10 per nine innings; even if he's not an ace, he seems likely to be a valuable starting pitcher on the cheap. Oh, and he's 21.
That's a lot to give away for this year's Edwin Jackson, especially now that draft compensation is no longer available for players acquired in midseason. At this point I'd rather see the Cardinals bring Miller up as a reliever for the rest of the season than watch them trade him for anything less than a clear number one.