If Matt Carpenter had two bad games in a row—went 0-10, say—his batting average would drop all the way to .318. Two bad games from Edward Mujica have sent his ERA from 2.15 to 2.53, and his closer's job from unassailable to positively assailed. New closers tend to seem solidly installed into the role right up to the moment they aren't anymore; suddenly we're left to remember that Fernando Salas's ERA back in 2011, when he lost the job, was somewhat lower.
These abrupt shifts in a pitcher's season are unnerving, but they're also what makes a closer-switch a much smaller deal in practice than it is in prologue. All the storyline build-up of a long season—this is Mujica's best year, this is a completely seamless transition from Jason Motte, this is a great way for Trevor Rosenthal to gain experience—has to be scraped off or retrofitted with new endings, and that takes a while. Somebody has to step forward a little more firmly than Mitchell Boggs did in 2011 when he got the first shot at it.
All of that takes a while. But once it's done, the actual change in the ninth inning is much simpler. The Cardinals have shown a willingness to experiment in the ninth inning for quite a while now; it's what brought Mujica there in the first place. It's what leaves them with these options:
Trevor Rosenthal's numbers are probably better than you think this year. He's striking out nearly 13 batters per nine innings, and as frustrating as his occasional control lapses have been, occasional is the key word. More stereotypical "closer" peripherals are hardly possible. And as for his theoretical career in the rotation—well, I'm still in favor of giving it a shot, but I'm not sure anything he and/or Mike Matheny do in 2013 could make it more or less likely at this point.
But if Mujica's problem is late-season fatigue, Rosenthal—the Cardinals' 23-year-old relief-innings leader—isn't a perfect replacement. His numbers have slipped from otherworldly to really-very-nice in the second half, and while he hasn't had a run like Mujica's he also hasn't pushed himself into consideration for the role. Which is why so much talk has focused on
who has. It's hard to talk rigorously about Kevin Siegrist, because it's so much more pleasant to just enjoy what he's doing while he does it; of course his peripherals (and everybody else's) don't support a 0.50 ERA, of course he will eventually have to deal with a batting average on balls in play over .200.
But having such a dominant left-hander around—one who's also young enough to be plausibly "shielded" from the full-time closer's job—seems like a convenient way into a more flexible bullpen, at least while the team figures out who it will ultimately rely on.
The Cardinals' bullpen isn't just partitioned by effectiveness—it's filled with players who are effective in different ways. Mujica and Seth Maness with low walk rates, Rosenthal and Siegrist (and Axford, maybe?) with high strikeout rates, Randy Choate with the platoon advantage. Some teams need to build roles for their relievers out of particular innings and statistics, but most of the Cardinals' pen seems to have the roles built right in. While they look for someone to anoint as closer, it might be worth organizing them based on what they do more than when they do it. (Even if it's just a distraction from the changing of the closer guard.)