The Red Sox have five major-league starting pitchers on their roster. That's about all we know for sure about the 2015 rotation. They sure are all major-league starters, aren't they? Capable of starting a major-league game, maybe even 30 of them per season each! They might have five guys who are just okay. They could have five guys who are all well above-average arms that lead them to victory far more often than not. Most of them could continue to (or start to) fall apart, leading the Red Sox to nothing besides further disappointment and a desire for the 2016 season.
While we can't exactly predict which of these it's going to be at this point, we can figure out how the Sox would get to any of these particular scenarios. The risks and benefits of these five pitchers are known, and throwing them all together to see what they look like in one place should be enlightening. With that in mind, let's look at the best-case and worst-case scenarios for each of Boston's five starters.
It wasn't that long ago that Buchholz was one of the game's top pitchers. He struggled a bit to kick off 2012, sure, but from 2010 through 2013, Buchholz posted a 135 ERA+ that ranked seventh in the majors over that stretch, in the same realm as Johnny Cueto, Cliff Lee, and Felix Hernandez. He had always relied on inducing fieldable contact, but in 2013, he started to miss bats consistently, allowing him to set a career-best strikeout-to-walk ratio and make his second All-Star team. That Buchholz could reemerge in 2015: last summer was plagued by the fallout from 2013's physical problems, and pitching in the playoffs and World Series likely didn't do him any favors as far as fully recovering goes. Further removed from those issues, maybe Buchholz is the top-of-the-rotation arm the Sox are looking for.
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The little omission from the above paragraph is that Buchholz's ERA+ ranked seventh-best from 2010 through 2013 minimum 500 innings pitched: Buchholz only threw 554 total innings during those four years, roughly 300 fewer than Lee and over 360 fewer than Hernandez. He was a lot closer to Cueto in terms of overall performance, where he was brilliant whenever he was on the mound but not on it often enough: you can glean some optimism from that if you wish, given how Cueto has become a more reliable and healthy pitcher of late, but 2013's injuries still loom large for Buchholz. It's possible that his most recent World Series run will have the same effect on him that 2004's did to Keith Foulke: he pushed himself a little too far physically, and now he'll never be quite right again. Flags fly forever, though.
That's probably pushing the issue a little further than it needs to, but you all saw Buchholz in 2014. He threw 170 innings, but most of them were miserable ones. The aggregate numbers for his time as a full-time big-league starter are still great, with a 114 ERA+, but Buchholz tends to play on the extremes: he's likely to be either excellent or a mess once again.
Porcello's 2014 is a career-year, but it's also not the first where he has been that good. Previously, the Tigers' infield held him back: 2014 was the first season of his career where his batting average on balls in play looked anything close to normal. His 2013 and 2014 campaigns look very similar, with an FIP of 3.53 followed by 3.67, and that gives hope to the idea that 2014 was more than just a positive blip.
Porcello isn't an ace, but he is a capable number two starter who could give the Red Sox 200 well above-average innings, especially when aided by a defense that includes Dustin Pedroia, Pablo Sandoval, and Mike Napoli.
Porcello relies heavily on grounders, so if holes in the defense emerge, there is little he can do to improve his work. If Pedroia is injured for a considerable length of time and the Red Sox aren't in a position to just put Mookie Betts in his place, or Xander Bogaerts fails to improve at all at shortstop, then there could be a major hole behind Porcello during his starts. His production isn't bad when the defense can't help him out, but he's far closer to an average-ish starter than a game-changing two when he's left to fend for himself. By itself this wouldn't be a huge problem, but remember: the Red Sox are focusing on a very ground-ball centric staff. If Porcello is dealing with defensive problems, then so is everyone else.
We've seen Miley's best-case performance before: between 2012 and 2013, Miley threw 397 innings with a 115 ERA+, one of just 15 pitchers in the entire game to throw that many innings at that level of quality over those two years. Things went a little south in 2014, but Miley was pitching in front of one of the worst defenses in the game, and he's a ground ball pitcher: the D-Backs were the third-worst in the majors at converting balls in play into outs, and Miley's batting average on balls in play shot from a normal .293 and .296 in the preceding two years to an untenable .317 in 2014 because of it.
In front of a Boston defense that should be significantly superior, Miley should be able to pitch like he did when he first came up, giving the Red Sox another arm capable of 200 innings and well above-average production in them. Like Porcello, he's a pitcher capable of putting up number two starter production, and is part of the reason the Red Sox aren't in desperation mode in their search for an ace. If the rotation is deep enough in quality, a high price for a rotation topper can be avoided.
BABIP is a bit fickle. Yes, it's possible (and even likely) that Miley's jump in BABIP can be attributed to the D-Backs' defense, but what if that's just noise, and the real problem is Miley's transition into a pitcher who can miss bats? Strikeouts are great and all, but maybe Miley's command isn't quite good enough to be that kind of pitcher, and it's helping leave too many pitches in places his opponents can get hits from. The good news is that Miley, like Porcello, can still be useful even if he's more of a mid-rotation arm than a two. The bad news is that the Sox rotation is built in a way that banks on both of them being twos, so anything less would be a problem that needs solving.
Masterson's 2014 was entirely caused by a knee injury that kept him from properly executing his mechanics. Without the ability to consistently hit his spots in the way he's capable, Masterson saw his ERA, walks, BABIP, everything shoot up, and the result was one of the worst seasons from a starter anywhere -- we're talking John Lackey 2011 levels of bad.
Like Lackey, though, if an injury was to blame, and the injury is no more, then Masterson can get back to doing what he should be doing, and that's getting batters out regularly. From 2011 through 2013, Masterson posted an ERA+ of 100 while averaging 205 innings per year. He was actually much better than that, as he produced a 122 ERA+ and 110 ERA+ in the years that flanked 2012, and started to miss bats in 2013 before the knee injury the next season erased that progress. Masterson could be yet another number two caliber pitcher, one who can throw 200 innings or more with an ERA+ anywhere between 110 and 120 or so. It all comes down to health, though, and whether he can replicate his success against lefties from 2013.
The knee is still junk, and so is Masterson's 2015.
Kelly has never been a full-time starter. He's started just 48 games over three seasons in the majors, with the rest of his work coming in relief. With that being said, his work as a starter has been solid, with Kelly posting a 3.44 ERA and averaging 5-2/3 innings per start. He's more of a back-end arm than anything, but since he's the fifth of five starters in Boston, that's precisely what he's been asked to be. He's reliant on grounders, but so is everyone else in this rotation: the Red Sox defense should help him, maybe even overselling his talent a bit, but that's okay. That's what defenses are for. If it all works out, the Sox will get 180 innings or so from Kelly -- his career high is 179, split between the minors and majors -- and while the bullpen will have to pick up the rest, he won't leave them too much of a mess to clean up in most of his starts.
Kelly has never been a full-time starter. He's started just 48 games over three seasons in the majors, with the rest of his work coming in relief. His work as a starter has been solid, but it's also come in a more forgiving league, more forgiving division, and more forgiving park. He's probably best-suited to be a swingman, and no one actually knows if he can succeed over a full season of starts against major-league hitters. If the defense behind him suffers, he doesn't miss enough bats to make up for it, and unlike Porcello and Miley, he doesn't limit walks enough to help himself out, either.
It's possible all five of these pitchers thrive: Buchholz is healthy which also means he's productive, Porcello and Miley bring the best of themselves from their previous teams, Masterson's knee is as sound as his pitching against lefties, and Kelly brings up the rear with the same kind of pitching that made him useful to the Cardinals. It's unlikely that everything goes Boston's way, and they could still add another starter to make Kelly more insurance policy than Plan A, but there is undeniable promise here. Even if it's clouded a bit by undeniable risk.