SEATTLE — Bruce Bochy has good incentive to live a righteous life, for if he winds up across the River Styx, he might be sentenced eternally to watch an endless loop of his pitchers walking the leadoff hitter in the late innings of tight ballgames.

It happened again Wednesday — Tony Watson was the culprit — and the batter predictably scored what proved to be the decisive run in a 3-2 loss to the Mariners that included another injury to a key player.

Brandon Belt hyperextended his right knee as he began a two-run tying rally in the seventh inning with an infield hit and hobbled off the field. He hopes the injury is minor, based on a lack of pain when trainers manipulated his leg to check for structural damage.

“I’m pretty optimistic,” Belt said.

Bochy said Belt will be down at least two days, but the medical staff will not know for certain until diagnostic tests are done in San Francisco on Thursday.

Again, the door to the training room revolves. Whenever one player is ready to return to the field — in this case, third baseman Evan Longoria — another takes his place on the table.

Longoria went 0-for-4, including grounding into a game-ending double play, in Triple-A Sacramento’s 5-3 loss to visiting Omaha on Wednesday night. Bochy planned to speak to executive vice president Brian Sabean and general manager Bobby Evans about activating Longoria for Thursday’s series opener against the Brewers at AT&T Park.

With 103 games gone, the Giants might have to stop fantasizing about the team finally being “whole.” That does not seem to be in the cards. But the Giants do have to feel fortunate they have gotten good help from the understudies.

“God (willing), nothing is serious with Belt,” pitcher Derek Holland said, “but somebody’s going to step in. That’s what I love about this team. Whoever comes here, whether it’s a pitcher or a batter, they’ve done their job, and that’s outstanding.”

Holland has been a strong understudy.

He was in the rotation, then booted out when Jeff Samardzija and Johnny Cueto returned, then shoved back in with Samardzija going on the disabled list for the third time.

On Wednesday, Holland held Seattle to two runs, one earned, in six innings, and dunked his ERA below 4.00 (at 3.92) for the first time this year.

For most teams that fancy themselves contenders, that would be enough to win. Not around here.

Coming off an 18-10 June, the Giants are 8-11 in July because their offense has gone missing, entering Wednesday’s game last in the National League in runs (68), homers (11) and OPS (.640) for the month.

Many of the core players are riding the worst slumps. That includes Belt, who was 1-for-33 when he started the seventh with a hard grounder that deflected off first baseman Ryon Healy’s glove to second baseman Dee Gordon, whose throw went into the Mariners’ dugout.

Belt collided with pitcher Mike Leake as he crossed the bag but was not sure whether that was when he hurt his knee. He didn’t feel it until he started to walk to second on the error.

After he came out, the Giants tied the game on softly hit but perfectly placed RBI doubles by Brandon Crawford and Hunter Pence.

It was 2-2 when Watson walked Guillermo Heredia to start the eighth. After a Gordon sacrifice, Jean Segura bounced Sam Dyson’s first pitch up the middle to bring the winning run home.

The Giants again failed to have their first winning road trip since Fourth of July week last year. They went 2-3 in Oakland and Seattle. Each of the last four games was decided late and by one run.

“You look at the last four games, it’s unbelievable how similar they are,” Bochy said. “This was another tough one for us.”

Henry Schulman is a San Francisco Chronicle staff writer. Email: hschulman@sfchronicle.com Twitter: @hankschulman