HOUSTON — On Monday afternoon, a day off between Games 2 and 3 of the American League Championship Series, a new Instagram story appeared under the account of Houston Astros third baseman Alex Bregman. Behind a caption saying “lil pregame video work” flashed clips of Bregman and two Astros teammates smashing back-to-back-to-back homers off right-hander Nathan Eovaldi, the Boston Red Sox’s designated Game 3 starter, during a game nearly four months earlier.

To watch those videos in private for motivation and/or a mental refresher was reasonable, even smart practice for a hitter about to face a semi-familiar pitcher. But to post them to more than 400,000 followers, including countless members of the media, said a couple of things about Bregman: that he was brash and cocky, and that he couldn’t fathom a scenario in which he would end up regretting the post — say, in the event the Astros lost and Bregman himself disappeared as an offensive factor.

And that, as it turns out, was a perfect description of Bregman, the face of the Astros this fall, in the stunned and sullen aftermath of their Game 5 loss to the Red Sox at Minute Maid Park on Thursday night, the one that sent them packing for the season and sent the Red Sox to the World Series: still brash, still cocky, still unable to fathom the end of the Astros’ hopes of defending their 2017 title.

“They beat us,” Bregman, 24, said through gritted teeth. “Do I think the better baseball team won? I don’t know. I wouldn’t have traded any of these guys for anybody over there.”

Many around the game, even many outside their own clubhouse, believed the Astros were the most dangerous, most flawlessly constructed and most formidable team in baseball right up until the moment, at 11:41 Central time Thursday night, when it was proved they weren’t. And even then, the Astros weren’t fully ready to admit what five games in October had settled.

“This team was better than last year’s team, I believe,” Bregman said. “But the ball’s got to bounce your way in the postseason. We’ll learn from it, and everyone in here will have a little bit of an edge, a little bit of a chip on our shoulder, knowing that we believe we should have been back-to-back champions.”

As evidence of the sport’s capricious ways, the Astros could point to all sorts of plays made and unmade, and calls that could have gone their way, but didn’t: the controversial fan-interference call in right field in Wednesday night’s Game 4 loss that took away a potential home run from Jose Altuve. The diving, game-saving catch that same night by Red Sox left fielder Andrew Benintendi, when a miss might have given the Astros the win. The high flyballs that seemed to die at the wall for the Astros but sail over it for the Red Sox.

“I said it last year: To win the World Series, so much has to go your way,” Astros pitcher Lance McCullers said. “Whether that means the team [for whom] it goes their way is better, or whether they got lucky, just got the bounces — baseball’s so funny, man.”

But it is also true that the Astros, at the worst possible moment, suffered system failures and individual stumbles that hadn’t happened in a long time, and that collectively led to four straight losses, including the last three at Minute Maid Park, a building in which they had gone 10-1 in four previous playoff series over the past two autumns.

In Games 2 through 4, the Astros pitching staff, which this season posted the lowest ERA (3.11) of any AL team since the introduction of the designated hitter, gave up 23 runs — more than in any other three-game stretch all season. Closer Roberto Osuna, who had pitched brilliantly since coming over in a July trade, gave up the pivotal grand slam to Boston’s Jackie Bradley Jr., the series MVP, that broke open Game 3. Right-hander Ryan Pressly, who hadn’t allowed a run since Aug. 10, gave up a critical one in the seventh inning of Game 4.

Collectively, Astros pitchers posted a WHIP of 1.099 in the regular season, the lowest of any team in more than 100 years. But in the ALCS, their WHIP shot up to 1.364.

In assessing some of those failures, Houston justifiably gave credit to Boston’s lineup. “They don’t concede any at-bats,” Manager A.J. Hinch said. “They never got off our fastball. They laid off tough breaking balls. They do it right. [And] they never stopped coming at you. They’re a relentless group.”

“The at-bats those guys were taking,” McCullers said, “were exhausting for us.”

Whether it’s because he drove the Astros’ lineup all season, or because he drove their media coverage, or because he was never afraid to say what the rest of his team was thinking, Bregman was conspicuous in the disappearance of Houston’s offense over the series’ final three games, during which the Astros scored just nine runs.

Up until Game 3 — or, if you prefer, up until his fateful Instagram story — Bregman had been having a postseason for the ages, hitting .417 with a .708 on-base percentage, a 1.000 slugging percentage, two homers and four RBI and reaching base 17 times in 24 plate appearances. But in those last three losses, he went 2 for 12, reached base just four times and drove in just one run.

“This game is all about failure,” Bregman said. “You’ve got to look in the mirror, get back up and get better.”

It was also clear the Astros were a diminished version of themselves in October. Second baseman Altuve, the 2017 AL MVP, played with an unidentified right knee injury that is likely to require offseason surgery. Shortstop Carlos Correa has been dealing with back and oblique injuries for much of the season. McCullers has an elbow injury that may also require surgery, and pitcher Charlie Morton has a shoulder issue that limited him to 15 innings in September.

“The heart of this team is really kind of unparalleled,” ace Justin Verlander said. “We had guys playing through so much. I’m not going to name names, but I think you could see some of it on the field.”

In an interview with Astros beat writers, Hinch compared the thought of sitting Altuve with “try[ing] to live with someone pulling your heart out of your body. We’re not going to live without Jose.”

The smart people who run the Astros, the industry’s most aggressively analytics-focused organization, will of course examine what went wrong and what steps to take, but they are unlikely to see the need for massive change and upheaval. While Houston’s pitching staff went from very good in 2017 to all-time great in 2018, its offense took a step backward this season, going from leading the league in runs scored and OPS in 2017 to finishing sixth and seventh in those categories this year. But much of that, they believe, was likely due to the injury-related absences of Altuve, Correa and George Springer.

Unlike last winter, the Astros face the possibility of significant free agent defections, with starting pitchers Morton and Dallas Keuchel, catchers Brian McCann and Martin Maldonado and super-utility man Marwin Gonzalez among those eligible to sign with the highest bidder.

“The unknown,” Keuchel acknowledged late Thursday night, “is a weird feeling.”

But the Astros also remain a player-development machine, with more future stars on the way. One of them was on display this month, with rookie Josh James, a former 34th-round pick with all of 23 regular season innings to his name, pitching significant innings in Game 4 and showing off a fastball that touched 102 mph.

In addition, their top five minor-league affiliates all made their leagues’ playoffs in 2018, with two of them winning championships. All five of those teams’ pitching staffs led their leagues in strikeouts.

The Astros are far from being finished as a force in the American League, but over five nights this month, in the face of a Red Sox team that may or may not have been plainly better, their aura of invincibility was pierced, and it wasn’t all about odd bounces and questionable calls and ill-fated Instagram stories. They were also outplayed, a notion that isn’t as easy to absorb.