SAN FRANCISCO -- When Logan Webb made his Major League debut at Chase Field last month, the 22-year-old right-hander became the youngest pitcher to start a game for the Giants since Madison Bumgarner.

Webb showcased his precocity that night, striking out seven over five solid innings to earn his first career win, but since then, the Giants have experienced typical growing pains with their top pitching prospect.

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Webb delivered another shaky outing on Wednesday night, surrendering four runs over 4 2/3 innings in the Giants’ 6-3 loss to the Pirates at Oracle Park. He has now failed to complete at least five innings in three of his first five Major League starts for San Francisco, posting a 6.75 ERA over 22 2/3 innings.

“It’s frustrating as a competitor,” Webb said. “I want to put the team in the best position possible to win, and I didn’t do that again.”

The Giants plan to use the final half of September to audition players who they believe could play bigger roles in 2020, making this a key evaluation period for Webb, who will likely be among the contenders for a rotation spot next spring. While the Giants believe Webb has the weapons to develop into an impact starter, he has produced inconsistent results in this embryonic stage of his career.

“It’s about being consistent with where you get the ball,” manager Bruce Bochy said. “The kid has got good stuff. He just made some mistakes.”

Webb, who gave up a career-high eight runs over 2 2/3 innings against the Cardinals on Thursday, struggled to command his power breaking ball and changeup against the Pirates, who scored twice on RBI singles from Adam Frazier and Kevin Kramer to take a 2-0 lead in the second inning.

Jose Osuna’s second leadoff double of the night set the table for another run for the Pirates in the fourth. Webb fell into more trouble in the following inning after Kevin Newman singled, stole second and scored on an RBI single by Colin Moran. He was removed from the game after issuing a two-out walk to Josh Bell and departed after throwing 81 pitches, 52 for strikes.

The Pirates produced seven hits against Webb, though only three of them qualified as hard-hit balls, according to Statcast.

“It’s just a couple of missed locations,” Webb said “When you do that, you tend to give up hits. I kind of think I did that again today, and it’s just frustrating.”

The Giants’ offense didn’t fare much better, producing only three hits, their fewest at home since July 19. They cut the deficit to 4-3 in the fifth on an RBI single by Jaylin Davis, an error by Newman and a bases-loaded walk by Mauricio Dubon, but Pirates reliever Michael Feliz struck out Brandon Belt and coaxed a popup from Stephen Vogt to scuttle San Francisco’s rally.

“You’re going to have nights like this,” Bochy said. “Sure, we don’t like them, but they’re going hard. You look at this series, the first game got away from us, but we did look flat tonight. There’s no getting around it.”