We've known for years that as time went on, an inevitable trend in baseball would be the blurring of the line between "starting pitchers" and "relief pitchers" into just pitchers. Starters don't go deep into games anymore, and that's largely by design, knowing all that we know now about how less effective the typical pitcher gets when facing a lineup a third or fourth time.
We know the game is catching up to the theory because we saw the Dodgers and Astros use the idea to great effect on their way to the World Series in 2017. We know it because Houston manager A.J. Hinch did something unprecedented in Game 7, pulling a starter with a shutout before three innings (for a non-injury or gimmick reason) for the first time in World Series history. We know it because the Mets and Rays have outright said it is part of their 2018 plan, and because the relief-pitcher market is the only part of this frigid Hot Stove that's moving.
"If the numbers are telling you that guys can't get through the third time of the lineup, then they shouldn't be out there," said Mets GM Sandy Alderson to MLB.com's Anthony DiComo in November.
Rays manager Kevin Cash echoed a similar thought at the Winter Meetings: "I think with all the information out there right now, it's pretty telling, we can find other ways to allocate those quality pitches, rather than the back-end pitchers."
It's not difficult to see what they mean, looking at the data. Batters have better outcomes the deeper pitchers go.
2017 Major League times through order 
SP 1st time: .314 wOBA
SP 2nd time: .332 wOBA
SP 3rd-plus: .340 wOBA
RP 1st time: .309 wOBA
wOBA is just like OBP, except it gives increasing value to extra-base hits rather than treating each time on base equally. The 2017 Major League average wOBA was .327.
What that means is that the idea of what a starter "should do" has to change. A short successful start shouldn't be seen as lesser than a longer, mediocre start, especially if you have another pitcher who can go multiple innings ready behind him. You'd probably think a little differently about Arizona's Taijuan Walker, for example, if he'd escaped 2017 with a 3.12 ERA (which he allowed through the first five innings) than the 3.49 he actually posted -- thanks to a 6.16 ERA when he was still in the game in the sixth inning and beyond.
But which pitchers are best suited for this kind of work? Maybe we can help. This is not a one-size-fits-all plan; no one is going to tell Max Scherzer or Corey Kluber to take a seat after the sixth inning in the regular season, nor should they. Instead, we need to find pitchers who have been successful the first time or two through a lineup before having their success be spoiled by damage accrued the third time or more, when a fresher pitcher may have been better prepared to get outs.
In order to find suitable cases, we set a minimum of 50 plate appearances against as a starter in 2016-17 against each of the first, second and third-plus times through the order. We also set a minimum wOBA of .320, or roughly the Major League average, the first time through. After all, if you're not good early on, none of the rest of it matters.
When we ran the search, high on the list were Houston's Charlie Morton and Brad Peacock, who each had career years while averaging fewer than six innings per start. They weren't asked to do more than they could offer, and the Astros were better for it. Who else could be good candidates?
Brent Suter, Brewers
Why: Because he's more deceptive than overpowering

There were 314 pitchers who threw at least 250 four-seam fastballs in 2017, and only three (including knuckleballer R.A. Dickey) had a softer average velocity than Suter's 86 mph. One of them was Kyle Hendricks (85.9 mph), so that alone doesn't make success as a starter impossible, and Suter was a nice surprise as a starter (3.45 ERA in 2017, and 3.95 in his brief career).
That's not why Suter tops this list, however. He's here because he's had such massive, drastic differences in outcomes as he turns the lineup over. Over the last two seasons, he's allowed a .240 wOBA the first time through -- an elite number that's well better than the starting pitcher average of .329 -- and an enormous .484 wOBA the third time and beyond.
We'll admit the sample here isn't huge, as he's faced only 59 hitters the third time and beyond, but he allowed more homers (three) than he's allowed in 235 hitters he's seeing the first time (two). And as a low-velocity arm with relief experience who pitches for a progressive organization, the conditions are perfect. He could be a valuable swingman for a handful of innings, and it wouldn't matter so much if those innings came to start or relieve.
Lucas Giolito, White Sox
Why: A hittable fastball could use the bullpen boost

Giolito, one of the prizes of last winter's Adam Eaton trade, was the third-highest name on our list, beyond Suter and Peacock, and he had a superficially good Chicago debut, with a 2.38 ERA in seven starts, though a 4.94 FIP, fueled by a high homer rate and low strikeout rate, didn't back that up.
While the rebuilding White Sox will give him every chance to start, his fastball wasn't the one that had been advertised as hitting 100 mph with relative ease as a Minor Leaguer in 2015. The fastball we saw in 2017 was 92.4 mph, just below the Major League starter average of 92.6 mph, and his four-seam spin rate of 2,122 RPM was also below the average of 2,240 RPM. A league-average spin rate can often mean a straight fastball without movement -- and he allowed a .539 slugging percentage, along with 10 of his 11 homers, off the fastball.
Again, he's young. He'll get his chances. But the fastball concerns, and the fact that there's been a 229 point difference in wOBA from the first time through to the second, point to at least having a move on the radar.
Dinelson Lamet, Padres
Why: Lack of a suitable third pitch

We actually just highlighted Lamet on our list of potential 2018 breakout pitchers, so it may seem odd to see him here, too, but that tells you a little about how wide the potential outcomes are here. Lamet has a strong fastball (95 mph) and a very good slider, which he used to be dominant (.154/.241/.296) against righty batters, but he had no weapon to get out lefties (.258/.365/.502).
It was the same the deeper he went into games. Lamet was strong the first time (.261 wOBA, compared to the second time of .287 wOBA), and got crushed after that (.411 wOBA). He'll need a usable third pitch if he wants to be a pitcher who can last for more than four or five innings.
Lance McCullers, Astros
Why: To help durability issues, and because he's an Astro

Remember when McCullers relieved Morton to throw four innings of shutout baseball against the Yankees to finish off Game 7 of the American League Championship Series? It was a taste of what could be, and he's on the right team to make it happen. Like Lamet, McCullers is a two-pitch pitcher, with nearly 90 percent of his pitches being a fastball or a curveball, and he's had trouble staying healthy, missing time in 2016 with shoulder soreness and hitting the DL twice in 2017 with back issues.
Like the others here, he's been far better the first time (.282 wOBA) and second (.322 wOBA) than the third and beyond (.347 wOBA). At age 24, he's still young, but the combination of having two pitches, health issues and a progressive team makes him a perfect candidate to be used judiciously.
Robert Gsellman, Mets
Why: Because his performance demands it

This could probably also be Seth Lugo, or Rafael Montero, or Zack Wheeler, or Steven Matz; the Mets have no shortage of talented arms who haven't earned the trust to go deep into games. But we'll focus on Gsellman, who had what looked on the surface to be a big step back (5.19 ERA) from a successful first look in 2016 (2.42 ERA).
Now, realize how his career .329 wOBA breaks down.
1st/2nd time through -- 305 wOBA
3rd/4th time through -- .393 wOBA
That's extreme, of course. You wouldn't expect it to be that large going forward. Yet as with the others, there's nothing that says Gsellman can't be a useable back-end starter for years to come. But what if he can be more than that? You'd certainly take the guy with a 3.52 ERA through four innings, as he has, and not want to see the guy who has allowed a 7.52 ERA after that. (In his short career, his velocity has dropped from 94.4 mph the first time to 93.4 the third time.)
The Mets are well-situated to stack a bunch of four-inning guys atop each other, in front of a good bullpen, and behind the "big two" of Jacob deGrom and Noah Syndergaard. Gsellman's not alone, here.
Nate Karns, Royals
Why: Because it's the only way he might stay healthy and productive

Karns is a hard-throwing righty who has never been able to stay healthy enough to last in a rotation, reaching 100 innings just once in parts of five seasons. In 2017, his season was cut short in July after thoracic outlet surgery, though he's expected to be ready for Spring Training. If that sounds like a perfect type for this kind of role, it should. Why worry about what a pitcher can't do (in this case, give you 32 starts and 200 innings) when you can focus on the good things he can do?
Over the last three seasons, one thing Karns has been able to do is miss bats and gets outs, at least at first. In his first two times through the order as a starter, he's collected a 25.1 percent strikeout rate and a .291 wOBA, both above average. Beyond that, the whiff rate drops to 20.2 percent, and the wOBA jumps to .378, which is how he's ended up with a 4.24 ERA since 2015. Karns is now 30 years old, and there's little evidence he'll hold up to a regular rotation workload. Why not lean into the skills he's shown? Used properly, he might be able to deliver on his considerable talent.