OAKLAND -- The day after pulling off the last-minute, surprising trade to land six-time All-Star right-hander Zack Greinke at the Trade Deadline, the Astros reeled off eight consecutive wins -- capped by a blowout of the Orioles a week ago -- to take over the best record in MLB. They appeared to be a freight train no one could stop behind a rotation that went 8-0 with a 1.89 ERA during the winning streak

A week later, a series of unforeseen circumstances has left the Astros’ pitching staff scrambling to fill innings and get outs. Monday’s rainout in Chicago pushed Greinke’s second start with the Astros back a day, leaving a hole in the rotation for Saturday’s game at Oakland. Gerrit Cole was scratched from his Tuesday start in Chicago with a strained hamstring, forcing manager AJ Hinch to throw a “bullpen game.”

• Box score

The Astros called up rookie Rogelio Armenteros from Triple-A Round Rock to start Saturday, in desperate need for him to pitch deep into the game, especially coming on the heels of Friday’s game in which Houston used six relievers in a 13-inning loss to the A’s.

It didn’t go well.

Armenteros was roughed up for five runs and seven hits in four innings as the A’s feasted on Houston’s battered pitching staff and handed the Astros their fifth consecutive loss, 8-4, at the Coliseum, bringing Oakland to within 6 1/2 games of the AL West leaders. The A’s will go for a four-game sweep Sunday. Armenteros was optioned back to Round Rock after the game.

“He threw a little better than the score’s going to indicate,” Hinch said. “They put together a lot of at-bats in the one blow-up inning [a five-run third]. I didn’t want to send him through the middle part of the order again when the game was still close, but he gave us what he could.”

Astros rookie slugger Yordan Alvarez did his best to keep his club in the game. He socked two more home runs, giving him 19 homers and 55 RBIs in his 52 games.

“I just keep making adjustments,” said Alvarez, who was 1-for-11 in the White Sox series. “There’s ups and downs throughout the season, but the results came through today.”

With four relievers having pitched three times in the previous four games heading into Saturday, the Astros’ bullpen was reduced to two rested relievers -- Chris Devenski and Collin McHugh. Devenski relieved Armenteros in the fifth and gave up three runs, McHugh threw two scoreless innings and Joe Smith worked the eighth in his fourth outing in five games.

“We’ve been through this before,” Hinch said. “This is nothing we haven’t conquered before, so we’ve got to play better with more winning baseball. This has been a really brutal trip result-wise, but we’ll get back at it tomorrow.”

The good news for the Astros is help is on the way.

Veteran right-hander Brad Peacock, who began the year in the rotation before going on the injured list with a sore shoulder, will be activated Sunday. That will give the Astros a fresh arm who can pitch more than one inning, if needed. Plus, Cole is progressing well from his hamstring episode and should return to the rotation sometime next week during a four-game series against the Tigers in Houston.

The Astros, at 43-15, have the best record in baseball at home and open a 10-game homestand Monday with Wade Miley on the mound. Aaron Sanchez is likely to pitch Tuesday, followed by Justin Verlander on Wednesday and perhaps Cole on Thursday, giving Houston a chance to reset its rotation.

“We just need to worry about one game,” Hinch said. “I don’t care about the rest of the rotation now. Tomorrow’s game is the most important game, kind of flush the first part of this series down and let Zack go out and have a really good start, and we’ll have a happy flight home.”