The Rays may or may not have ushered in a "new era" when they started reliever Sergio Romo on back-to-back days against the Angels over the weekend, but what they surely did was prove that the idea had merit. Romo, facing a righty-heavy Halos lineup, struck out six of the nine batters he faced in 2 1/3 scoreless innings. After he dispatched with the quality top half of the Angels' lineup, Ryan Yarbrough and Matt Andriese were able to enter to start with the weaker bottom half.
If Romo had gone out and gotten hit hard, it would have set the idea back by years. You might never see a team try it again. But he didn't. It worked. Now the relevant question isn't "will we see this again?" it's "who is going to do it next?" It won't just be Tampa Bay. Not now.
"I'm intrigued to see how it's going to continue to work, because I'm confident we're going to do it," said Rays manager Kevin Cash. "It might not just be Sergio. It might be Jonny Venters. It depends."
It's not as simple as "take a reliever and throw him out in the first inning," however. Tampa Bay's strategy worked because it satisfied three important conditions.

1. Romo is very strong against righties, and easily hit by lefties.
No team has given fewer plate appearances to lefties in the first four spots in the order this year than the Angels have, almost guaranteeing he'd face righty hitters. By doing it this way, you're getting the best possible version of Romo -- the one that's allowed a .194/.263/.387 to righties since the start of 2017, not the one that's allowed a .272/.372/.438 to lefties.
2. The Angels are one of baseball's weakest-hitting lefty teams, leaving them unable to adapt.
If the Halos had moved up struggling lefty Kole Calhoun (.161/.199/.203) to ensure Romo faced a lefty and thus ended up giving Calhoun more plate appearances than Michael Trout, Justin Upton or Andrelton Simmons, that's a big win for the Rays in and of itself. (Notably, Shohei Ohtani was unavailable as a hitter on both days.)
3. The Rays don't have five strong starters.
Tampa Bay has had, at times, a three-man and four-man rotation. A team like the Astros, for example, who are on a potentially historic rotation run, wouldn't bother trying to do this. It's not worth it.
It has to make sense on both sides, is the point. Houston won't bump Justin Verlander to start Joe Smith, no matter what the numbers say. No one is going to do this against a balanced top of the lineup like the Cubs have, with lefties Anthony Rizzo and Kyle Schwarber, righties Kristopher Bryant, Albert Almora Jr. and Javier Baez, and switch-hitter Benjamin Zobrist. And it has to be the right kind of reliever -- you're never going to see an elite closer like Craig Kimbrel doing this.
So which are the next best matchups to watch? Let's try to repeat the Rays' thought process.
The Angels were really the perfect candidate, loaded as they were with righty batters and without a good lefty hitter to move up.
Highest righty plate appearance percentage in the lineup's top four spots
Angels (97 percent)
Astros (95 percent)
Orioles (88 percent)
Cardinals (72 percent)
You can see why the Angels were so appealing, but they're not the only ones. Save for the rare appearance from Josh Reddick, the top Astros' four spots are primarily handled by righties George Springer, Alex Bregman, Jose Altuve and Carlos Correa. Since the start of 2017, they've hit .297/.367/.495 (.367 wOBA) against righties, and .320/.403/.535 (.397 wOBA) against lefties. It's not a huge difference -- they're great regardless -- but it's something.
Perhaps more interesting are the Orioles, who head to St. Petersburg this weekend to face the Rays, and the Cardinals. Baltimore has baseball's weakest group of lefty hitters, thanks in part to Chris Davis hitting .166/.241/.272. Much like the Angels with Calhoun, if starting a righty motivates the O's to give Davis (or Jace Peterson, or Chance Sisco) more plate appearances than Manny Machado, Adam Jones or Trey Mancini, all the better.

The Cards, meanwhile, are the only team in baseball without a lefty hitter within even 10 percent of league average, thanks to the struggles of Matt Carpenter and Kolten Wong. You start a tough righty to attack Tommy Pham, Marcell Ozuna and Jose Martinez, and you almost hope it makes them keep Carpenter (.194/.321/.351) up high.
Though Cash said Venters was an option, this might not work as well for lefties. The teams with the highest concentrations of lefty batters atop the lineup are the Braves (who now have righty Ronald Acuna Jr. hitting second behind switch-hitter Ozzie Albies) and the Indians (who have switch-hitters Francisco Lindor and Jose Ramirez around lefty Michael Brantley and righty Edwin Encarnacion).
The Rangers might be an exception, given that without Adrian Beltre and Elvis Andrus, they don't have a single righty hitter near league-average. Then again, Nomar Mazara is their only healthy league-average hitter of any hand, so the American League's weakest offense may struggle regardless.
So we know which teams might be most affected by this. Which teams are best suited to take advantage?
It should probably be an AL team (or a National League team playing at an AL park), given that they won't have to worry about running out of pinch-hitters with today's short benches. Then again, it doesn't have to be. If an NL team has to have a pitcher hit, it is no worse off than before. They could have a better-hitting pitcher on a day off, like Madison Bumgarner, take some swings. It doesn't rule them out.
There's not necessarily a scientific way to do this, or a perfect approach. The way we chose was a simple one. We eliminated the Astros, obviously; they're the only team with five healthy, unmovable starters.
Then we looked at righty relievers, specifically those who (since the start of 2017) have shown above-average performance against righties. (Remember, Romo isn't a great pitcher against all hitters, but Romo against righties is a pitcher you want.) We defined that as having faced at least 100 hitters, with a wOBA below .290. (The average righty reliever on righty batter wOBA in that time is .305).
We knocked out closers who will never do this, like Kenley Jansen, Kimbrel and Brandon Morrow, a few who are currently injured or having poor seasons, like Tommy Kahnle, Darren O'Day and Pat Neshek, or high-leverage setup men like Joe Kelly or Archie Bradley. We're still left with kind of a large list. That's sort of the point, though. As nice a career as Romo has had, he's not terribly unique. It's not hard to find relievers like him. Almost every team should have one or more like him.
This isn't meant to be a fully exhaustive list. You'll certainly find other candidates. Our best bets to do this soon, however, are …
AL
Romo (Rays)
Chad Green, Player Page for David Robertson (Yankees)
Matt Barnes (Red Sox)
Dennis Tepera, Seunghwan Oh (Blue Jays)
Yusmeiro Petit (A's)
Jose Leclerc (Rangers)
Thomas Pressly, Addison Reed (Twins)

NL
Peter Moylan (Braves)
Steve Cishek (Cubs)
David Hernandez (Reds)
Edubray Ramos, Luis Garcia (Phillies)
Craig Stammen, Kirby Yates (Padres)
Josh Fields (Dodgers)
Paul Sewald (Mets)
John Brebbia (Cardinals)
Cory Gearrin (Giants)
Again, not a full and comprehensive list. But if the soft-tossing veteran righty Romo can start against a top of the lineup that includes Trout, why couldn't Moylan? Or Cishek?
As you can see, there's plenty of reliever options, and we're only looking at righties. You could probably come up with a good matchup for every day for the remainder of the season.
We're not going to do that, however. We're going to look for what could or should happen right now, such as whether the Blue Jays ought to follow the Rays' lead against the Angels this week. Instead of starting lefty J.A. Happ on Tuesday, why not start with Tepera or Oh?
Looking ahead to next weekend, Romo and the Rays repeating the trick against the Orioles seems almost too perfect not to happen. The Yankees welcome the Angels to the Bronx, and may have the benefit of an Ohtani-free lineup, given that he's likely to pitch. Why not start Robertson?
We don't know who will do it next, or where. We just know it will happen soon, because the temptation of turning a decent reliever into a good one against a known lineup where pinch-hitting is almost ceratinly not going to happen is tempting. People have been talking about this idea for years. The Rays finally made it happen.