SAN FRANCISCO — Despite their team’s fading playoff hopes, just one win in their first five home games and the two lowest attendance figures of the season over the last two days, nearly 28,000 Giants fans showed up Thursday to see the reigning Pitcher of the Month for August make his first start of September.

You should hope that there will be more opportunities next year.

Blake Snell fought through 42 pitches to end the first inning and never came out of his dugout, but the Giants were still able to avoid being swept by the Diamondbacks 3-2 thanks to relief work from Landen Roupp and key hits from Patrick Bailey.

Bailey hit a two-strike fastball from Diamondbacks closer Kevin Ginkel to left-center field, allowing Tyler Fitzgerald to run home from second base and score the catcher’s second RBI hit of the afternoon, marking Blake Snell’s briefest appearance in a Giants uniform.

Snell gave up two runs in the fourth-shortest start of his career, and his defense or inability to find the strike zone didn’t help him. He allowed two walks, both of which led to scoring position thanks to mistakes by the Giants’ defenders. Fitzgerald airmailed a throw to first base and Bailey dropped a strike three that would have ended the inning.

With his pitch count in an awkward position, the bullpen door opened early in the second inning and Roupp mowed through the next four innings in the longest and most effective outing of his young major league career. The 25-year-old rookie limited Arizona to one hit and two walks while making five strikeouts and allowing no runs.

The Diamondbacks didn’t put another runner in scoring position until Christian Walker opened the eighth with a double against Camilo Doval and Ryan Walker left him standing on third base with a strikeout of Jake McCarthy after Bob Melvin called his new closer to get the last out of the inning.

Snell’s agent Scott Boras was one of those who was in the house for the cloudless matinee at 21 degrees.

But he was on hand to announce the Giants’ six-year, $151 million contract extension with another of his clients, third baseman Matt Chapman, who put Michael Conforto in scoring position with a double and crossed the plate on Bailey’s two-out, two-RBI poke to left field, scoring the Giants’ only runs in seven innings against Arizona starter Merrill Kelly.

Like Chapman, Snell has the option to opt out after this season, and he is expected to use that opportunity to earn a long-term contract. Although he lasted just a single inning Thursday, Snell’s ERA in 11 starts since returning from a groin strain on July 9 is a sparkling 1.42, the lowest in the major leagues by a half-run differential.

Boras prefers his clients to set their price on the open market, and Snell is likely no exception like Chapman, who lobbied publicly and privately for a deal. He recently rebuffed Jordan Montgomery’s criticism of how Boras handled his free agents last winter when neither got the long-term deal he wanted.