NEW YORK — The trouble for Yordan Álvarez began after a 4-for-4 day in Anaheim on Sept. 22. The Houston Astros rookie looked like a veteran the way he drew 20 pitches from the Angels and stroked each hit to the outfield. One of his two doubles soared 388 feet to the center-field wall.

This was nothing new for the newcomer, who emerged as a strong candidate to win the American League Rookie of the Year Award for his .313 batting average, 27 homers, 78 RBIs and 53 extra-base hits in 87 regular-season games.

Then, like an autopsy determining the time of death, it appears that Álvarez’s performance unexpectedly collapsed.

Following Sept. 22 and eight games into his first postseason, Álvarez mustered three hits in 23 at-bats (.130 average). His pitch selection went sideways in a hurry. He became weaker at the plate. He went from scorching the ball at 95-plus-mph off his bat half of the time to, suddenly, doing so only one-third of the time.

The average major-league home run traveled 400 feet this season. Álvarez, a lefthanded Cuban Paul Bunyan, smashed his homers 11 feet farther. Then the power went poof. He has not homered and only has two RBIs since his banner day in late September.

His strikeout rate rose from 25.5 percent to 44 percent from the regular to the postseason, meaning that during the American League Championship Series against the New York Yankees, it has been almost a coin flip whether Álvarez will put the ball in play.

On Thursday night at Yankee Stadium, Álvarez showed a sign of life. With Alex Bregman on first, Álvarez pulled in his hands and muscled a single off a high fastball from the previously indomitable Chad Green. It was only Álvarez ’s second postseason hit off a pitch faster than 89 mph. Carlos Correa followed with a three-run home run that demoralized the home crowd.

Green’s heater aside, it has seemed troublesome how easily pitchers in the playoffs are blowing the ball by Álvarez. Fastball usage against him is up to 45 percent from 29 percent in the regular season.

The Yankees’ much-used bullpen led the AL with the highest average fastball velocity at 94 mph. New York’s relievers have reached back for a little extra, closer to 95 mph, in matchups with Álvarez in the ALCS.

He went from a secret weapon in the summer to a stymied threat in October.

“He is a guy we’re certainly aware of in the lineup,” Yankees manager Aaron Boone said on WFAN (New York) radio. “Controls the zone with big-time power.”

The designated hitter who’d cozied up to the cleanup spot in 32 starts (with a 1.144 OPS) was dropped to sixth in order by Game 3 of the ALCS because the opposition erased him from what had looked in the regular season like baseball’s deepest lineup.

“We’ve been able to pitch him pretty tough,” Boone said. “Hopefully we can continue that, because we know he can put a mistake in the seats.”

But the Yankees and the Tampa Bay Rays in the division series made several mistakes, and Álvarez seized only some of them. Pitchers have thrown 22 pitches in the middle horizontal plane of the zone — at the level of Álvarez’s waist, where he’d slugged .716 in the regular season — but he’s converted them for only four hits. He has swung and missed three pitches — all fastballs — left in the dead center of the strike zone, which before October was something he’d done only five times in the four months.

Worse for the Astros, who took a 2-1 lead in the series heading into Game 4, Álvarez has been abysmal against lefthanders lately. He’s gone from hitting .307 off them to swinging wildly at their out-of-reach pitches. Lefties have held him to 2-for-12 in the playoffs, and he likely will face more of the same challenges in Game 5 on Friday at Yankee Stadium against starter James Paxton. In their three matchups this year, Alvarez is 0-for-2 with a walk.

Álvarez saw nearly 1,500 pitches in the regular season. He appeared to have a preternatural gift for understanding his strike zone, often passing up borderline pitches that were called in his favor. He is swinging and missing 5 percent more often in the playoffs, which is not a significant increase. The real problem is that he is chasing balls that he used to recognize as harmful. Opponents have attacked him lower and more away — farther off the outside edge of the plate — and he has taken the bait. His whiff rate on those pitches has risen from 14 to 26 percent.

Yankees pitches surely have realized that one out of every four low pitches can flummox Álvarez. After striking out in Game 2 on a breaking ball from lefty Zack Britton that finished in the opposite batter’s box, Álvarez sledgehammered his bat, breaking it against the ground. He’d snapped, as well.

While a bat is as expendable in the majors as a paper straw, a hitter’s compass is difficult to hone quickly if it’s broken.