The Astros came into Monday’s game with a crew of mostly anonymous players, the worst record in the league, and the worst pitching staff in the majors with an earned run average of 5.51.

The Yankees had won four games in a row, and the Astros had lost four straight to the Red Sox in Boston, all by at least four runs. These were two teams with widely different aspirations on the season, but for one cold, gloomy night, it was all in reverse.

The Astros pounded Andy Pettitte and the Yankees, 9-1, in their first game at Yankee Stadium since 2010, and improved — modestly — to 8-18.

It was the Yankees first loss since Pettitte fell to the Tampa Bay Rays in St. Petersburg last week in his first loss of the season. On that night it was one critical pitch in the fifth inning that cost Pettitte the game — a pitch that Ben Zobrist hit for a two-run double. But Monday, it was a series of bad pitches that did in Pettitte.

In his worst start of the season, Pettitte gave up 7 runs and 10 hits in four and a third innings against the only other team he ever played for.

His control was off from the beginning as he allowed three runs in the first inning and two more in the fourth, all with two outs. Catcher Carlos Corporan had a two-run double in the first and a two-run homer in the fifth against relief pitcher Adam Warren.

Corporan collected four hits, a career high, and reached base five times, thanks to Eduardo Nunez’s error in the ninth. Corporan appeared to injure his left leg while running the bases in the top of the seventh and was limping noticeably, but he stayed in the game.

Brandon Barnes, the right fielder, also three hits and three runs batted in, both career highs.

Pettitte had won his first three starts of the season before the loss to the Rays and was pitching almost as well as ever. Despite that loss in Florida, he still came into Monday’s game with a 2.22 E.R.A. But too many of his pitches were up in the strike zone, away from the intended target and easy prey even for the young Astros. His breaking balls had little bite and seemed to roll tantalizingly in front of their hitters, who sprayed them to all fields.

Austin Romine, called up Saturday to replace the injured Francisco Cervelli, made his season debut, but did not fare particularly well. He went 0 for 3 at the plate, and was hit by a pitch in the abdomen in the seventh inning. He remained in the game.

Astros starter Lucas Harrell, meanwhile, held the Yankees to one run and eight hits, struck out four and walked one. Even before he took the mound he had a three-run lead, making his evening much easier.

Pettitte got two outs in the first before Brandon Laird, the former Yankee draft pick, singled to left. That was followed by another single from Chris Carter and a run-scoring single by Carlos Pena. Pettitte then walked Ronny Cedeno before Corporan ripped a double to right, pushing home two more runs, and before long the rout would be on.

The Astros looked pretty good defensively, too. They had turned the most double plays coming into the game with 31, and the Yankees, not surprisingly considering how this game went, handed them four more.

INSIDE PITCH

Kevin Youkilis did not play Monday because of stiffness in his back, which has become a lingering issue for the Yankees, who must decide whether to place him on the 15-day disabled list. But Youkilis played on Saturday, which means that if the Yankees place him on the D.L., he could not return until May 13. If he had not played on Saturday, he could have gone on the D.L. April 21 and been available on May 5. Joe Girardi said he had no regrets about putting Youkilis into that game. “No, none whatsoever,” he said. “The player said he was ready to go and we put him in.” Youkilis had a magnetic resonance imaging test Monday, and the Yankees were awaiting the results.