When Seth Maness joined the team, I flagged him as truly one of the weirdest prospects in baseball. He had a long history of success in the minors, despite a below average strikeout rate, and because of a microscopic walk rate and an astonishing groundball rate. He was a weird prospect, who was going to put to the test a pretty much unheralded skillset.
Not that he's the first control artist/groundballer to appear in MLB, but almost no one had the preposterous 9+ K/BB rate he had in the minors, and certainly no one did so while striking out a below average number of batters. Nobody had a good sense of how he would do, because there wasn't really anyone with that extreme an array of talents.
After almost 30 innings, Maness tentatively looks like a success, with the caveat of the 30 innings we're talking about.
He's keeping up the miniscule walk rate. Of the 5 walks he has issued in those 30 innings, only 2 were unintentional. The intentional walks give him a 4% walk rate, but his unintentional walk rate is more around 1.8%. Going into last night, he was ranked 12th among all pitchers with at least 20 innings pitched. Teammates Edward Mujica and Adam Wainwright check in at first and third on this list, in case anyone wants to note a big factor in the success of the Cardinals' pitching staff. Unfortunately, there's not an easy way to compare unintentional walks, as best I know. Although that walk rate is not reliable given the sample size (it's getting close, though; he's faced 121 batters, and walk rate stabilizes at 175), his walk rate is completely in line with his minor league numbers.
The other big story of Maness's success is the groundball rate. He has a 70% groundball rate; as of last night, he was second in the majors among the hundreds of pitchers who have thrown 20 innings or more (Brad Ziegler has him beaten by a little). I don't even know what to say about that. Even Jake Westbrook doesn't throw 70% groundballs. Unlike his early walk rate, his groundball rate HAS reached the point where it becomes sustainable.
You can't argue with the results. Maness has a 2.73 ERA, a 3.37 FIP, and a 2.92 xFIP. For those who are interested in such things, he has an enormous SIERA-xFIP split (two stats which normally track each other very closely), driven largely by that unearthly ground ball rate. Compared to his 2.92 xFIP, he has a 2.12 SIERA. Good god, man.
The other thing that one should note about Maness - and part of his remarkable utility in a fireman role - is that he generates an astonishing number of groundball double plays. Of the seven PAs with the highest WPA for Maness, SIX resulted in inning-ending double plays. He has generated nine double plays in his 121 batters faced. Only starters Jake Westbrook (12) and Adam Wainwright (17) have generated more double plays; Westbrook has faced twice as many batters (249) as Maness, and Wainwright has faced more than four times as many (547).
That ability to generate a double play makes Maness very valuable out of the bullpen, especially in light of his 13.8% K rate, relative to the 21.4% league average. Not many guys with a 90 mph fastball do so well in relief.
I don't want to oversell Maness; he's one of many relievers performing very well. Trevor Rosenthal and Edward Mujica are almost certainly better relievers. But both of them come with more conventional reliever skills. If Maness isn't the best reliever, he is surely the weirdest.