NEW YORK -- It's just like any other day, and it's nothing like any other day. As the New York Yankees and Houston Astros packed their gear and headed to the airport for a 3½-hour overnight flight, they said all the right things -- that they'd done this before, that this was old hat, that it was just like any other getaway day. And in a sense, they weren't lying. Packing and flying and playing the next night are all familiar undertakings.

Of course, taking that tack ignores the context of this particular undertaking, and the context happens to make Saturday's outing unique in the annals of baseball history. Never before has a team facing elimination this deep in the MLB playoffs started a game with a relief pitcher. The Yankees will do that after their season-saving 4-1 victory Friday night. They won't be the only ones, either. In Game 6 of an American League Championship Series that the Astros still lead 3-2, they too will lean on an opener, a concept that is less than two years old and already finds itself so widely accepted that a pair of 100-plus-win teams will rely on it in a seminal postseason game.

The rapid evolution of baseball landed the game here, where bullpen games -- long the domain of lazy spring-training afternoons in which managers simply wanted to get guys an inning -- are now seen as far superior to using a traditional starting pitcher. Both teams have multiple options to start. Both are sticking with reliever-heavy game plans anyway, even amid a stretch in which they'll play four games in four days, a rarity in October.

"We're gonna get in. We're gonna try to get some sleep. Get up. And pretend it's a regular situation," said Yankees starter -- er, long reliever -- J.A. Happ. "We're gonna do our best. Because that's the only way to do it."

The 2019 ALCS: Fake it till you make it!

Happ was just being sincere. All season the Yankees and Astros have flown into the wee hours of the morning, slept off their fatigue and made it to the yard the next day for a game. This is how players live. They just don't do it in the midst of a series that is verging on epic, particularly if the Yankees can parlay one of their strengths all season -- their power relief corps -- into a winner-takes-all Game 7 with Luis Severino primed to take on Houston's Gerrit Cole, behind whom the Astros haven't lost since mid-July.

So, no, it's kind of not a regular situation. It's true the Oakland Athletics tried to bullpen the 2018 wild-card game against the Yankees, so it has been done. But not a bullpen vs. bullpen battle, and certainly not one with the season at stake for one team and elimination-game potential for another.

The entire spectacle is chaotic and larded with deliciously strategic bonbons ripe for consumption. Take the choice of opener. Do the Yankees go with their likely candidate, Chad Green, who opened 15 of the 17 times New York utilized one this season? Or does Aaron Boone, their manager, save Green for a higher-leverage spot later in the game, knowing that one of his late-inning options, Tommy Kahnle, already has pitched in back-to-back games?

Houston's options are even more interesting. "My guess was Brad Peacock," Astros reliever Will Harris said, "until the eighth inning." That's because Peacock pitched the eighth during Game 5 and was the only Houston reliever to appear. Consider, though: Peacock is a former starter, so he is familiar with and comfortable pitching the first inning. His fastball-slider-changeup mix looked righteous in retiring Brett Gardner and the top two hitters in the Yankees' lineup, DJ LeMahieu and Aaron Judge. And the Astros adore his competitiveness, having seen Peacock finish the final 3⅔ innings of Game 3 in the 2017 World Series.

Asked if he would be ready to go in Game 6 after finishing Game 5, Peacock said: "Absolutely. If they ask me, I'll do whatever it takes. They know if they ask me, that's what I'll do."

And if he does it, Peacock will join an illustrious list of men nicknamed Firpo, Three-Finger and Doc to finish one postseason game and start the next on zero days of rest.

In 1924, Firpo Marberry struck out one batter to save Game 2 of the World Series for the Washington Senators and the next day tossed three shaky innings in a loss. His predecessors were Three-Finger Brown for the Chicago Cubs in the 1910 World Series and Doc White for the Chicago White Sox in the 1906 World Series.

Other options exist for Astros manager AJ Hinch if he fears Peacock gave LeMahieu and Judge too good a read of his pitches in their at-bats. Because Justin Verlander plowed through six more innings after a miserable first that wound up costing Houston Game 5, Hinch rested all of his non-Peacock relievers. Closer Roberto Osuna, setup men Harris and Ryan Pressly, the fast-slow combination of Josh James and Joe Smith -- all of them together, plus Jose Urquidy, the rookie starter who can provide the so-called bulk innings if need be, constitute a force. Even better for the Astros is that if Game 6 is trending toward failure, they can give their best relievers an extra day off in preparation for Game 7, when they would feature arguably the best starting pitcher on the planet at the moment -- provided Cole doesn't pitch in Game 6, which Hinch deemed "unlikely."

Boone doesn't have quite that luxury, though he finds himself in a tenable position for now. His three best relievers, closer Aroldis Chapman, setup man Zack Britton and Kahnle, all pitched in Game 5. Beyond that Cerberus is a group that is lacking Boone's confidence (Adam Ottavino), potentially wild (Jonathan Loaisiga), more of a specialist (Tyler Lyons), a mop-up type (Luis Cessa), Happ and Ben Heller.

"I was pitching in intrasquad games in Tampa a couple of days ago," Heller said, "and now I'm pitching in the ALCS."

Heller, who threw 7⅓ innings this year in his return from Tommy John surgery, replaced the injured CC Sabathia on the Yankees' active roster. And in a bullpen game, for which a single out against a right-handed hitter might be needed, Boone could summon Heller, against whom right-handers are hitting .163 in his career.

The beauty of the bullpen game is the Ben Hellers of the world can find themselves in the highest-leverage situation. As much as it is about the players -- it's always about the players -- managerial chess is a fascinating game to play in October, and Boone vs. Hinch will be scrutinized with great fervor.

One wrong move could cost the Yankees their season.

One slip-up could leave the Astros an anomalous Cole start away from an all-time meltdown.

"It's both teams trying to get outs and trying to do it the best way we can," Boone said. "We've had some -- obviously a lot of guys go down this year and different people not available. We'll just do our best to piece it together."

Until then, both teams planned to go about their nights as usual. Urquidy would fall asleep on the plane. Peacock would play hearts with teammates until they landed. Happ would stay up, trying to get comfortable in the seat that never seemed big enough for him. And about 15 hours later, come 8:07 p.m. ET, someone would throw Game 6's first pitch without any intention of throwing the last.

The bullpen game is here. Chaos awaits.