Giants’ bullpen collapses again in loss to Padres

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SAN DIEGO — For the reigning champs, it’s no longer about calculating magic numbers and critiquing games behind. It’s about shutting down wounded players and dreading elimination day.

The season is closing fast on the Giants, who lost to the Padres 5-4 Thursday night in a game Madison Bumgarner — amid stare-downs with umpire Joe West — was denied his 19th win by a worn-down bullpen.

Bumgarner gave up three runs on three hits in seven innings and exited with a 4-3 lead, only to watch Hunter Strickland and Sergio Romo permit the tying run in the eighth and Mike Broadway and Josh Osich yield the winning run in the ninth, Alexi Amarista’s single giving the Padres a walk-off win for the second straight night.

Wednesday, Jake Peavy’s six scoreless innings were wasted as manager Bruce Bochy set a franchise record by using 10 pitchers in a nine-inning game.

“It’s a crazy game we play,” Bumgarner said. “We’ve been through a lot this year. A lot of injuries, a lot of adversity. I feel real good about the group of guys we have here. The way we played this year, right now it’s not what we would have wanted and worked for. At the same time, you’ve got to realize how hard this game is.”

Bumgarner’s confrontations with West were clear to see, especially when West pulled off his mask and either barked at or glared at Bumgarner, who wasn’t thrilled with the strike zone when he was both hitting and pitching.

Madison Bumgarner #40 of the San Francisco Giants pitches during the first inning of a baseball game against the San Diego Padres at Petco Park September 24, 2015 in San Diego, California.

Madison Bumgarner #40 of the San Francisco Giants pitches during the first inning of a baseball game against the San Diego Padres at Petco Park September 24, 2015 in San Diego, California. Photo: Denis Poroy, Getty Images Photo: Denis Poroy, Getty Images Image 1 of / 1 Caption Close Giants’ bullpen collapses again in loss to Padres 1 / 1 Back to Gallery

“Joe was behind the plate?” asked Bumgarner, showing his serious look. “I don’t pay attention to the umpires. I didn’t even realize he was back there.”

All three runs off Bumgarner came on Derek Norris’ home run. The lefty has two more scheduled starts, and Bochy said he’d speak with his ace about whether he’d pitch beyond Tuesday’s game against the Dodgers.

Brandon Crawford became the fourth Giants shortstop to hit 20 home runs, and Matt Duffy stole his 10th base, becoming the third rookie in franchise history with at least 70 RBIs, 70 runs and 10 steals.

But the offense wasted early opportunities. The Giants had eight hits in the first three innings, and Angel Pagan took a unique turn around third before he was thrown out at the plate.

Buried in the National League West behind the Dodgers, who can clinch as early as Saturday, the Giants decided it was time to shut down several players, including Hunter Pence (oblique strain) and three others with concussions: Brandon Belt, Nori Aoki and Gregor Blanco.

“I would be shocked if any of them would play this year,” Bochy said.

Pence hasn’t played since Aug. 17 and will finish the season with just 52 games played. Bochy agreed the loss of Pence impacted the team like the absence of Buster Posey in 2011. Posey played 45 games in ’11, and the Giants went 86-76, finishing eight games back.

This year’s Giants are 79-73.

“I think it’s similar,” Bochy said. “It’s played a role in a lot of these games not to have Hunter Pence in the lineup.”

As for getting eliminated, it’s not a matter of if, but when. It could come this weekend in Oakland, as early as Saturday’s Tim Hudson-Barry Zito game. The jolt would be felt across the Bay Area if the A’s knock out the Giants for the first time since the 1989 World Series.

Or it could come in the ensuing Dodger series. The Dodgers clinched playoff spots on Giants turf in 1977 and 2006.

John Shea is a San Francisco Chronicle staff writer. E-mail: jshea@sfchronicle.com Twitter: @JohnSheaHey