...or otherwise titled "Who Gets Better, Who Gets Worse". The following is a relatively brief (Update: okay, so it didn't quite work out that way), oft-numeric summary of how the pitchers have performed so far, and whether they will get better or worse as the season progresses.
Things to know (if you're a stathead like me, skip this paragraph): CERA and ExpERA are two measures of what a guy's ERA should be, based on his peripherals so far (walks, homers, etc). Coincidentally, Dave posted a neat little piece on Julio Mateo just this morning which uses ExpERA, and you should definitely give that a look. DERA refers to the defense-independent ERA a guy should have, achieved by isolating how much he's been helped or hurt by the guys behind him and regressing his numbers towards the mean. BABIP, as many of you know, is a guy's Batting Average on Balls In Play. The league average typically hovers around .290-.295. LD% is Line Drive Percentage, which is pretty self-explanatory. Thanks go to the Hardball Times for sharing this particular data. League average: .177 (17.7%).
Will get better. Although this isn't exactly a fivesome of worldbeaters, if they continue to pitch as they have so far this year, we could expect their collective ERA to drop closer to 5.00. They've been hurt by disproportionately bad pitching in run-scoring situations (runners on, RISP, things of that sort), which tends to balance out over a full season. They've also gotten very little help from the defense, which is turning 72.3% of balls in play into outs for the entire staff, but 70.8% for the starting rotation. They are giving up incrementally more line drives than the league average, but the difference is small, so that shouldn't be held responsible (line drives, understandably, are more difficult for a defense to turn into outs, so LD% is directly related to BABIP). Their GB/FB ratio of 1.16 is also right about on the average, so that's not to be blamed, either. The bottom line: it's an underwhelming group of pitchers, to be sure, but the starters are better than their numbers would indicate so far, so even if we left the same five guys out there for the rest of the year, their total ERA would improve.
Will get worse. The group has looked pretty incredible so far, but a lot of that has to do with the defense turning a remarkable 75.6% of balls in play into outs. There's no reason to expect that to continue - they're allowing a lot of line drives (more than the starters, if you can believe it), and even though the group is pretty flyball-oriented, that doesn't come close to explaining a BABIP that low. Over the next four-odd months, you'll see more balls dropping in and more balls clearing the fence, as the bullpen's ERA approaches 3.75-4.00 territory (barring any personnel changes).
Will get better. Moyer's been both quite unlucky and reasonable fortunate at the same time - on the one hand, the defense has been awful behind him, but on the other, he's done a better-than-expected job of limiting the damage by pitching pretty well (comparitively) with men on base. Even if he keeps getting hit as hard as he is right now for the rest of the season, the hits will start finding the gloves more often, and Moyer's ERA will inch back around the 5.00 mark.
Will get better. Batting average against with none on: .226. Batting average against with RISP: .311. While Gil hasn't been anything fantastic this year, he's been unusually bad in important situations in which he's excelled in the past. The defense has been pretty standard behind him, and the line drives are around the average, so don't expect too much variation in those departments over the rest of the summer. He'll pick up the strikeouts and reduce the hits allowed in run-scoring situations, though, which will make him a more successful pitcher.
Will get worse. He's a flyball pitcher in a run-repressive environment, so he should post a pretty good BABIP, but nothing that low. The culprit? An extraordinarily-low amount of line drives, quite literally half as many as he allowed a year ago. Even if he's become a better pitcher, with a good idea of how to keep hitters of balance, such a low LD% is unsustainable (Johan Santana's LD% last year: .162), so the line drives will come, and thus so will the hits. Which is a shame, because Franklin's been the only dependable pitcher in the rotation so far. Pray for more strikeouts.
Will get better. But will he have the chance? I doubt it. You don't need numbers to see that Sele's getting hit hard these days, but that isn't enough to explain such a high BABIP on this team. It's possible for a pitcher to compensate for an underachieving team defense - see any Yankee arm for the past, oh, 63 years - but Sele's issuing a bunch of free passes without missing any bats, which, combined with the defense, is something of a triple whammy. Nevertheless, his ERA could be expected to descend to something around 5.50 if given a chance to finish the year. Let's hope he doesn't get it.
Will get better. Everything's been failing Joel so far - his command, his defense, his home runs - but those will improve over the summer. He's been brutalized with men in scoring position to the tune of a .395 BA, which won't continue, and he's allowed twice as many home runs as you'd expect, given his amount of fly balls surrendered. He's going to come back from his ten-day vacation a better pitcher, and the Mariners organization will get tons of credit for taking action to turn around another struggling young arm, but the truth of it is that a lot of his perceived gains are just going to be luck evening out. Which isn't to say that there isn't a mechanical issue here, because that could be the explanation for his low velocity of late. There's another possible explanation, but I'd prefer not to mention it right now.
Will get worse. Read Dave's post if you want to know why. Mateo could continue to be the same pitcher through September as he's been through the first month and a half of the season, and his ERA would still rise into the mid-3's. There's no way around it - he's gotten way more help from his defense than you could reasonably expect, and he's been quite fortunate keeping balls in the yard. These won't continue, and neither will his freakishly low ERA.
Will get worse. In fact, the only reason his ERA looks so shiny right now is because three of the four runs he allowed on April 9th were unearned (E-4), even though they all scored on home runs. Eddie's always shown a propensity to limit hits on balls in play, but rarely to this extent, and he has a bit of a home run problem which guarantees elevated ninth inning blood pressures for the rest of the year. Barring further rotator cuff deterioration, he should still be a good pitcher, but his ERA should jump a few ticks.
Will get worse. Stop me if you've heard this one before: he's getting way more help from the guys behind him than you'd expect, particularly for a pitcher who puts more balls on the ground than the rest of the bullpen. He's not a guy who gets a lot of strikeouts, so the fact that he's hitting his spots pretty well and limiting home runs will keep him reasonably effective, but the hits are going to start coming. Even in his terrific 2003 campaign, Hasegawa still had a BABIP of .250, so as long as he keeps throwing strikes, he should be pretty good. Just not this good.
Will get better. He's like the reverse Julio Mateo - he's been hit hard, and the balls have dropped in nearly 40% of the time. What's surprising is that none of these well-hit balls have sailed over the fence yet, something with which Villone has struggled in the past. It's unlikely that he continues to avoid the longball, but it's even less likely that he keeps up this kind of hit rate, particuarly if he's used as a textbook LOOGY. Look for an ERA in the low- to mid-4's by the end of the year.
Will get worse. Putz is just another guy in this bullpen to have a crazylow, unsustainable BABIP, and as a fairly good groundball pitcher, we can expect it to climb more than Mateo's or Hasegawa's over the course of the year. The good news is that JJ's home run issue doesn't date back into his minor league career, so while he's going to allow more hits, not too many of them will leave the park, which will help keep the damage to a minimum. Watch his strikeout rate - as that goes, so does Putz's standing as a quality setup man. It's going to be interesting to see if his early gains stick around.
Will get worse. You talk about a guy who's been creamed - more than a third of the balls put in play against Nelson so far have been line drives. The good news is that there's no way that's going to continue; the bad news is that he's still got sloppy control which will prevent him from returning to his previous glory days. What he's done so far is pretty close to what we can realistically assume he'll do for the rest of the year - it pains me to say he'll get worse, because I don't think is ERA is going to rise very far from where it is now, but it's more likely to increase than decrease, so I'm erring on the side of probability. Would you believe that Nelson's never given up more than seven homers in a season?
Will get better. (Who's going to be the one to say "it certainly can't get any worse"?) Thornton hasn't been hit really hard, which makes his five home runs allowed seem anomalous. And they are, because so far he's allowed home runs on 26% of fly balls, which is more than twice the normal expected rate. That will certainly come down, along with Thornton's ERA. His walks will never make him much of a high-leverage pitcher, but his strikeouts will keep him from embarrassing himself. If you remove two home runs from his total so far, his expected ERA drops by nearly a run and a half.