Is this the best rotation in Diamondbacks history?

This was originally going to be a "Isn't Patrick Corbin doing well?" piece, looking at the best seasons, coming out of the blocks, by a Diamondbacks starting pitcher. But when I pulled up the list of those, I was struck by something else. There have been ten occasions where a qualifying Arizona starter has had an ERA below 3.00 through the first 35 games: three of them have happened this season, with not just Corbin, but also Trevor Cahill and Wade Miley meeting the standard. Here's the table of all such performances.

We hadn't had one pitcher get off to such a good start since 2009 - and now we have three? Got to be impressed. Now, I wouldn't be so sacrilegious as to claim that Corbin and Cahill are as good as Randy and Curt. But in both 2001 and 2002, the seasons we had them going full-bore, there was a steep drop-off after Johnson and Schilling. For instance, in our World Series winning campaign, the rest of the rotation had a 4.97 ERA; the following year, it was a little better, but the back end still came in at 4.59. It's easy to forget that in 2001, our #3 starter by innings pitched was Brian Anderson, who went 3-9 with a 5.46 ERA over 22 starts.

Obviously, the 2013 back end has had their issues, with Ian Kennedy and, in particular, Brandon McCarthy, struggling over the first month and a half. But the bottom line, across the entire rotation, is a 3.80 ERA through Thursday, the best put up by the Diamondbacks since 2002, when the figure over the entire season was 3.67. It's an improvement of almost half a run over last season (4.26). Though worth noting, we have not had our depth tested at all this year, and Reno numbers have been disappointing, from the two who "competed" with Corbin this spring. Tyler Skaggs has a 6.89 ERA and Randall Delgago is even worse, at 8.80.

So, it does seem that sustaining our 3.80 might be tough, requiring either unexpected health from the current members of the rotation, or matching performances from whoever comes in to replace them. On the other hand, we do have Daniel Hudson, who has a 3.58 ERA over 53 starts for the Diamondbacks; he is looking to return, perhaps around July. If performances continue the way they have, when that happens, it's going to give Kevin Towers and Kirk Gibson a very interesting decision in terms of the roster, and who gets bumped.

We should remember, the baseball landscape has changed a lot since 2001. That year, the National League ERA was 4.36, more than half a run above what it has been to date in 2013 (3.81). To take that into account, we can also look at the fWAR numbers from the rotation, which also values things like innings pitched [if you want a full explanation of pitcher WAR, grab a sandwich and go nuts] Here are the results for each season since the D-backs started, ordered by fWAR, with a host of other stats (some not available until 2002). I've scaled the counting stats (like wins and innings pitched) for this season to a full 162-game schedule - those are shown in italics.

That would dial back enthusiasm for this year's model, though it still ranks among the best rotations of the past decade, trailing 2008. A couple of other numbers stand out: we're on pace to get the most innings out of our starters since 2002, which seems to destroy any hypothesis that our starters aren't going deep enough into games. And a good part of the reason for this year's success is the lack of home-runs allowed: the lowest rate in team history. That's why our xFIP is higher, because that number would be expected to regress to the norm, and take our ERA with it, though a good ground-ball rate will help, by reducing the number of balls with a chance to leave the park.

What's particularly impressive is that it's so highly-ranked with one starter owning an ERA above five, and another north of seven. So, while regression from Corbin seems likely, if Kennedy and McCarthy can get back to their career numbers (3.85 and 4.17 respectively), then maybe we'll be able to have this discussion again, deeper into the season.

[All statistics are up to and including May 9]