What's wrong with Shelby Miller? It's the question of the moment in Arizona, as Miller has struggled so badly (an 8.59 ERA and just 11 batters retired in his past two outings) that his short starts have led to an overworked bullpen and caused the team to even consider having him skip a start in order to work out his issues.
Needless to say, it's not exactly what the D-backs were expecting when they sent a huge talent package to Atlanta -- rightfully ignoring Miller's 6-17 record on a 95-loss Braves team in favor of a 3.02 ERA better than that of Chris Sale or Johnny Cueto -- in what was last offseason's most hotly debated move. Instead, he'll face his old St. Louis mates on Tuesday night in Phoenix with a lot to prove, and possibly with his spot in the rotation on the line. Will the hard work he's put in between starts pay off?
The good news: There are three easily identifiable issues. The bad news: If it were as easy as simply snapping your fingers, surely Miller would have already done so.
1. His mechanics are off
We start here because it's the obvious concern, given the fact that scraping his hand on the mound has been an issue in each of Miller's past two starts. But while any injury Miller may have incurred from the bruising can't be discounted, it hardly seems like the whole issue, either, since he told MLB.com's Steve Gilbert that his issues were "more about poor mechanics" than anything.
That makes sense, because the data backs it up. After years of hitting the strike zone 53-54 percent of the time, Miller is doing so just 46 percent of the time this year. Just look at Miller's horizontal release point of his fastball last year as compared to this:

The definition of horizontal release point has to do with the left/right location of the pitcher's release point, but the specifics are less important than the obvious change from Atlanta to Arizona -- Miller's first three starts were completely out of sync, which could explain a lot about his troubles with command. It's a good sign that his most recent start seemed to be closer to normal, but hat's hardly the only thing: Miller's Statcast™ extension is about three inches longer this year -- another indicator that he's just not releasing the ball like he did last year.
2. His velocity is down... sort of
With St. Louis, Miller was regularly in the 93-mph range with his fastball. With Atlanta, that stepped up to 94.1 mph. With Arizona, so far, it's just 92.9 mph. Now, some of that is unsurprising. We know his mechanics are off. We know that he slammed his hand into the ground. We know that many pitchers start off with lower velocities until the weather warms up (though that's less of an issue for Miller, since his starts have all been in Arizona or California).
Given that everyone involved claims that Miller's arm is healthy, it seems fair enough to assume for the moment that the velocity could be fixed if the mechanics are. There's good news on this front: Miller's first two starts: 92.3, 92.3. His last two: 93.4, 93.7.

3. He's dropped a useful pitch
In 2013 with St. Louis, Miller used his four-seam fastball 74 percent of the time. That was too much, as he learned from teammates Adam Wainwright and Justin Masterson in 2014, so he dropped it to 62 percent and added a sinker and a cutter. Miller took that with him to Atlanta and used the four-seamer and sinker an equal 33 percent of the time, helping him to set career bests in both home-run rate and ground-ball rate.
It made sense. Last year, Miller's four-seamer averaged 93.8 mph and 2,258 rpm, both pretty close to league average. As he told FanGraphs last month, "When you're only throwing a four-seamer, guys see it and see it and see it. I think you have to mix it up." The mix of three fastballs helped keep hitters off balance.
So far this year? The four-seamer is back up to 68 percent. The sinker is almost nonexistent, down below five percent. Obviously, the walks are an issue, but Miller is missing fewer bats (from a 19-percent to a 13-percent strikeout rate) and inducing fewer grounders (from 48 to 40 percent). Last year, he had a very good exit velocity against of 87.7 mph; this year, it's up to 94.7 mph, the second highest of any regular starter, behind only the Braves' Bud Norris.
It's not clear why. If it's simply wanting to go back to the comfortable four-seamer as he tries to work out the command problems, that's understandable and (likely) temporary, though since it's Miller's first year on a new team, it's possible that he's been asked to drop the sinker -- which would be concerning.
The takeaway to all this? Nothing seems unfixable. It doesn't seem to be a health issue, and if Miller turns it around quickly to pitch the rest of the year as he did with Atlanta, no one will remember this, since the D-backs have managed to start 11-10 without much from him anyway. There's time to turn it around, and there are some small signs that he's on the verge of doing so. The Cardinals won't make it easy on Tuesday night, though. They never do.