Out of nowhere, the Houston Astros got a huge head start on October.

On a dizzying day that featured two dozen trades, the Astros pulled off the biggest and most startling deal, adding ace Zack Greinke to an imposing rotation already loaded with All-Stars Justin Verlander and Gerrit Cole.

Plenty of familiar names were on the go Wednesday — Shane Greene and Mark Melancon boosted the Braves’ bullpen, with Scooter Gennett, Jesús Aguilar and Mike Leake among those also moving.

The Boston Red Sox made no moves.

But it was the Astros’ acquisition of Greinke from Arizona for four minor leaguers that quickly became the talk of baseball. The deal came right before the deadline for swapping players to still have them eligible for the postseason.

“We had him high on our list and we didn’t know this was even remotely possible and it really wasn’t until the last 48 hours and really the last 24 hours that we started to get traction on something,” Houston General Manager Jeff Luhnow said.

The AL West leaders and 2017 World Series champions added two other pitchers, too, getting starter Aaron Sanchez and reliever Joe Biagini from Toronto.

“Houston made some big deals. They’re really good. They were good before,” Red Sox President of Baseball Operations Dave Dombrowski said.

A lot of contenders were busy.

The Chicago Cubs added Detroit’s Nicholas Castellanos to their lineup, the Phillies got outfielder Corey Dickerson from Pittsburgh and the Washington Nationals added relievers Daniel Hudson, Roenis Elías and Hunter Strickland.

“When it comes to trades, one thing I’ve learned is, just wait,” Cubs Manager Joe Maddon said. “You’ve got to wait until the very end and it plays itself out. The 11th hour is the most powerful hour there is. To get things done before that, it normally doesn’t work to get what you want. There’s the 11th hour at work.”

Several players whose names swirled in the tradewinds stayed put.

Giants ace Madison Bumgarner, Mets starters Noah Syndergaard and Zack Wheeler, and Pirates closer Felipe Vázquez remained in place.

“Nothing changed for me. I never expected to be somewhere else until that happened,” Bumgarner said. “I just have a job to do and I’m going to do it. We’re going to miss a few guys we got rid of. That’s going to be tough.”

Major League Baseball made July 31 a hard deadline this year for trades. Now, no deals can be made until after the World Series.

“This was a unique deadline, it felt,” said Yankees GM Brian Cashman, whose AL East-leading team didn’t make any significant moves.

Pitchers Marcus Stroman, Andrew Cashner, Homer Bailey and Jason Vargas were among the players who were traded in recent weeks.

And on Tuesday night, the Cleveland Indians agreed to send pitcher Trevor Bauer to Cincinnati in a three-team swap that brought back outfielder Yasiel Puig. That trade became official Wednesday, setting off a full morning and afternoon of swaps.

YANKEES: The Yankees placed slugger Luke Voit on the 10-day injured list with a sports hernia that may require surgery.

Voit was pulled from New York’s 4-2 loss to the Arizona Diamondbacks on Tuesday night and went for an MRI, which revealed the injury. Manager Aaron Boone said the team is still evaluating the severity.

The Yankees expect to determine in the next day or two whether Voit will require surgery, which would take him out of the lineup for about six weeks. If he doesn’t need an operation, Boone said it will be a “day to day scenario.”

Right-hander David Hale was also placed on the injured list with a lumbar spine strain, retroactive to Sunday. He became the 24th Yankees player on the IL this year, four more than their disabled list total last season.

Infielder Breyvic Valera and right-hander Jonathan Holder were recalled from Triple-A Scranton/Wilkes-Barre.

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