"Best" is a bit relative, in that mediocrity can be considered the best when all you've dealt with is inadequacy. But, progress is progress, and the Royals' rotation, for all the gnashing of teeth over the James Shields/Wil Myers trade, has progressed into what might very well be the best rotation they've had in years.
The 2012 Royals featured three pitchers with over 100 innings: Bruce Chen, Luke Hochevar, and Luis Mendoza. Jeremy Guthrie finished fourth on the club in innings, despite not making his first appearance in a Royals' uniform until July 22. There's one obvious issue there -- a lack of durability -- but there's a second, too, in that no Royals' starter was good enough to reach the 200-inning threshold, and many weren't even good enough to reach 150 or even 100.
It's not a new problem, either. 2011 featured Luke Hochevar and Jeff Francis as the innings leaders, and neither was close to average. Kyle Davies, Sean O'Sullivan, and Vin Mazarro received opportunities, and not all necessarily because injuries forced the Royals to go those unappealing routes. Go back a little further, and the presence of Zack Greinke and (healthy) Gil Meche takes the sting off, but the underlying issue is the same: there hasn't been any meaningful rotation depth or quality for years.
The last above-average pitching staff the Royals fielded, by ERA+, was in 2007, and that had more to do with a fantastic bullpen than a rotation that featured Odalis Perez (5.57 ERA), Jorge De La Rosa (5.82), Scott Elarton (10.46), and Davies (6.66) combining for 69 starts. To make things more depressing, the last average staff prior to that came in 1997, and that again was mostly the pen with a dash of Kevin Appier thrown in. You have to go back to 1996, when Tim Belcher, Jose Rosado, Chris Haney, and Appier all posted above-average ERA+ -- and three of the four logged over 200 innings -- for the last above-average Royals' rotation.
You might not agree with shipping off Myers and others for Shields, but you can at least understand the sense of urgency given the last productive Royals' rotation was 16 seasons ago. Given the Shields acquisition, retaining Guthrie, and the Ervin Santana trade, the Royals have set themselves up to snap that streak of awfulness.
Shields gives the Royals the kind of arm they haven't possessed since the Greinke trade, one capable of throwing well over 200 quality innings. Shields struggled with batting average on balls in play and the long ball in 2010, but has changed his approach a bit since to incorporate grounders more often. The result? Shields has tossed 477 frames in the past two seasons, with a 120 ERA+, 3.6 times as many punch outs as free passes, and he even has 14 complete games to his credit. He might not be a true ace, one of the half-dozen or so best hurlers in the game, but Shields can certainly top a rotation given his abilities.
Guthrie is a mid-rotation starter who has averaged 194 innings per season over the last six years, topping 200 frames three times in that stretch. He doesn't miss a ton of bats, but he has a career-long habit of reducing BABIP thanks to the contact he induces, and it's made him an above-average pitcher. Things got a little rough as a Rockie, but Guthrie's rebound with the Royals is closer to the truth. He'll likely produce an ERA in the low fours, and toss around 200 innings doing it. That's no ace, either, but simply being capable is something the Royals have had a distinct lack of over the years.
Santana is the most-significant question mark of the bunch, given how poorly he pitches when his slider is off, but like the other two, he's usually durable, and a rebound season would mean a lot to this rotation. In fact, Shields, Guthrie, and Santana rank fifth, 15th, and 18th in innings since 2007. The only other Royal, former or otherwise, to make even the top 70 is Greinke -- Guthrie and Santana might not be the best around, but they're significantly better than their predecessors, and have shown themselves capable of pitching regularly, too.
There's one other thing to watch out for. The Royals' defense has been terrible for years, as they've finished in the bottom half of the league in Defensive Efficiency every season since 2001, often closer to the worst than the middle. Part of that has been poor defenders, but a portion could also be the quality of pitching. While BABIP tends to sit around the league average, that assumes that all of the pitchers in question merit a major-league career. The lower in the minors you get, the higher BABIP is, as baseball's version of natural selection causes promotions and releases to occur, winnowing out those who wouldn't conform to BABIP standards in the big leagues. Some of that is on the raw defense behind the pitchers, but it also has to do with arms who lack the stuff, the strategy, the command, etc. to succeed at higher levels.
The Royals have rolled out starter after starter over the years who look more Omaha- than Kansas City-bound, and it's very likely that hurt their defense. If these three starters can pitch like they have in their careers, and the Royals manage to fill out the back-end of their rotation with actual, MLB-caliber arms, then we might see one of the better Royals' defenses of the past few years as well.
Will it be enough to justify the trade, or make the Royals competitive? It's hard to say at this point, especially with so much relying on the lineup's ability to grow and develop, or Wade Davis' potential to convert back to starting successfully. But, there have been some first steps here towards fixing a very long-term issue for the organization, and the results could be better than many expect.