OAKLAND -- With 129 games elapsed, the A’s must admit that they’re not sure what will happen after their starter yields the mound to the bullpen.

The A’s perplexing bullpen faltered again Sunday, as former closer Blake Treinen yielded Evan Longoria’s two-run single in the seventh inning that propelled the Giants past Oakland, 5-4.

• Box score

The Giants (65-65) captured three of four games against the A’s to secure The Bridge trophy, which is given to the team that wins the annual confrontation of Interleague cross-Bay rivals. Oakland (74-55) remains in the thick of the American League Wild Card race, but the loss magnified its inability to improve the club's footing by subduing a perceived weaker opponent such as San Francisco.

Fortunately for the A’s, they’ll receive another chance to dominate an also-ran when they visit Kansas City for four games beginning Monday. Yet outclassing the Royals will be an essential task for Oakland, since its K.C. stop precedes a three-game weekend visit to Yankee Stadium.

The A's relievers must be ready for challenges, not just against the Royals, but also at the latter-day version of the House That Ruth Built.

“We still feel like we have a really good bullpen,” Oakland manager Bob Melvin said. “We just have not pitched to our capabilities. At times, it looks like we’re getting things settled, and at times, not. So we just continue to grind.”

This should have been a satisfying Sunday for the A’s. They conducted a 30th anniversary celebration for their predecessors who swept the Giants in the 1989 World Series. They announced that Dave Stewart, the ace of that ‘89 staff, will have his No. 34 jersey retired next year.

However, the current A’s allowed the Giants to rally, something that the champion A’s rarely permitted.

With two outs in the fourth inning, Oakland’s Mark Canha belted his second home run of the afternoon to snap a 3-3 tie and put the A’s ahead, 4-3. The score remained intact until the seventh, when San Francisco loaded the bases with nobody out. Brandon Crawford reached base on first baseman Matt Olson’s fielding error. Then Jake Diekman, currently the leading lefty in Oakland’s bullpen, walked Donovan Solano and hit Mike Yastrzemski with a pitch.

In came Treinen to face Buster Posey. Treinen won that showdown by striking out Posey, though he needed 11 pitches to do so. Longoria proved to be much more economical, swinging at the first pitch and grounding it into left field.

“I got a ground ball. I can’t control where it goes,” said Treinen, who Melvin has relied upon in high-leverage setup situations.

Regarding the options facing Melvin when he must pick up the hotline to the bullpen, Treinen said, “It's our job to get outs. I don't think anybody is thinking twice [about roles]. This season's been what it's been. And probably not the easiest for Melvin to dictate where guys are going to throw. All of us understand that.”