SAN FRANCISCO -- The Giants entered Monday night 55-0 when leading after eight innings this season, a testament to the dominance of All-Star closer Will Smith and the back-end depth of the bullpen. 

A spate of injuries is now testing that depth, as a depleted relief corps couldn’t protect a two-run lead in a 6-4 series-opening loss to the Pirates at Oracle Park. With Smith unavailable due to back tightness, the Pirates rallied to score four runs in the ninth inning against Tyler Rogers, Fernando Abad and Jandel Gustave, spoiling a strong start from left-hander Madison Bumgarner.

“A shame we couldn’t hold on to get him a win there,” manager Bruce Bochy said. “But a nice job he did. You look at the body of work, he’s been a savior.”

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Bumgarner limited Pittsburgh to two runs over seven innings and departed with a 4-2 lead, but he came away with a no-decision after the Pirates staged their late comeback. Rogers pitched a scoreless eighth and returned to the mound for the ninth, but he was removed after giving up a leadoff single to pinch-hitter Kevin Kramer.

Bochy then summoned Abad, who retired Cole Tucker on a groundout before walking Josh Bell and yielding a single to Elias Diaz to load the bases. Kevin Newman followed with a two-run single off Gustave, tying the game at 4. Former Giants prospect Bryan Reynolds, who was traded to the Pirates in exchange for Andrew McCutchen last year, delivered another stinging blow, driving in the go-ahead run with a single to center field.    

Bochy’s high-leverage relief options have dwindled considerably in the second half, as the Giants moved Mark Melancon, Sam Dyson and Drew Pomeranz at the Trade Deadline and have now lost Reyes Moronta (torn labrum in right shoulder), Trevor Gott (right elbow strain) and Tony Watson (left wrist fracture) to arm injuries in recent weeks. 

Smith, who is the lone member of the Opening Day bullpen left on the Giants’ current active roster, hasn’t pitched since Friday and couldn’t get his back to loosen while throwing Monday, forcing Bochy to lean on newcomers who had largely delivered before stumbling against the Pirates.

“I’d put him at day to day,” Bochy said of Smith. “He’s still pretty tight. He tried to throw today, and he just couldn’t go. We’ll re-evaluate him tomorrow. Meanwhile, we’ll try to piece this thing together and see if we can get through it.”

The bullpen collapse cost Bumgarner a chance to overtake former teammate Matt Cain for the most wins by a pitcher at Oracle Park, where he’s logged a 2.82 ERA this season. His 60 career home wins remain tied with Cain, but he’ll likely have two more chances to break the record over the final three weeks of the season.    

“Obviously, I’d like to have [a healthy bullpen], but I feel like the guys we’re throwing out there are still pretty good,” Bumgarner said. “Tonight just didn’t go our way.”