If Ryan Jackson and Pete Kozma were pitchers, the Cardinals would have switched them out for each other months ago. That's not about the Cardinals—it's just bad pitchers can pull off the band-aid incredibly quickly. Mitchell Boggs, who I swear played for the Cardinals this year, was worth -1.3 wins above replacement level in 15 innings; Pete Kozma has been worth -0.2 WAR in 132 games. (Compared to an average player Kozma's finally caught up to Boggs.)
That gap between average and replaceable makes intuitive sense. No matter how bad Pete Kozma can get, a nightmare start for him is five outs, spread out over nine innings, and maybe a booted grounder; Mitchell Boggs' troubles began in a game where he allowed six earned runs and recorded one out. Pitchers, especially relievers, are able to demonstrate their replaceability for us all at once.
That—and the way they're carried on the roster seven at a time—has created flexibility, and that flexibility has been absolutely crucial for the Cardinals this year. Seth Maness and Kevin Siegrist, neither of whom was first in line for a job at the start of the year, have combined to throw 91 innings with an ERA of 1.68.
They've buried relievers, too, of course; Carlos Martinez never seemed like a serious candidate in the bullpen, and Fernando Salas has thrown two medium-leverage innings since late April. But in all they've given 13 relievers at least 10 appearances.
At shortstop the Cardinals have, to their credit, shown some of that flexibility; Daniel Descalso was an avowed non-shortstop who's now played 42 games there. But having made their novel moves—to punt on offense with Kozma and defense with Descalso—they've been less swift to respond to what doesn't work out.
In a little less than a half-season of starts at shortstop over the last three years, UZR has Descalso worth -24 runs on defense per 150 games, and I'm willing to buy that. He's the guy we worried Skip Schumaker would be. I'm glad they tried it, but now—well, now they've tried it. They're getting no value and no new information from continuing to try it.
Ryan Jackson is more Keith Butler than Kevin Siegrist. The Cardinals don't love his defense, and they don't like his bat, and he's coming off a brutal slump of his own, and he's the only option—they can't cycle from the Keith Butler of Shortstops to the Michael Blazek of Shortstops to the Seth Maness of shortstops. But the lesson the bullpen's taught us is that there's little to gain from hanging on to the replacement-level guys they have now to the exclusion of guys who haven't yet given us an exhaustive look at that replaceability.