SAN FRANCISCO — Curt Casali wasn’t supposed to be in the Giants’ starting lineup on Monday night, but he found himself pressed into catching duties after Patrick Bailey was scratched with right side tightness.

Casali ended up providing a much-needed spark with his bat, delivering an RBI single that kicked off a four-run rally in the fifth inning and propelled the Giants to a 5-3 win over the MLB-worst White Sox in Monday night’s series opener at Oracle Park.

The Giants trailed, 1-0, before their sluggish bats finally broke through against Chicago right-hander Jonathan Cannon in the fifth. Thairo Estrada, who returned from the injured list on Monday after missing 21 games with a left wrist sprain, and Grant McCray strung together back-to-back singles to put a pair of runners on with one out for Casali, who lined another base hit to center field to tie the game.

“He swung the bat really well tonight,” manager Bob Melvin said. “He didn’t try to do too much, shooting the ball the other way. It was a big hit for us at the time. Guys like him, a lot of times you’re not expecting to play, you come in and certainly contributed on both ends.”

The Giants went ahead on Tyler Fitzgerald’s RBI double and then added on behind LaMonte Wade Jr.’s RBI single and Heliot Ramos’ sacrifice fly to center field. It was the type of rally that had long eluded San Francisco, which entered Monday batting an MLB-low .164 with a .489 OPS with runners in scoring position since July 30.

Casali has appeared in only 24 games with the Giants this year, but he made the most of his unexpected starting opportunity, adding another single in the seventh to secure his first multihit game since June 18.

“It was a tremendous feeling for me,” Casali said. “I needed one of those. I haven’t hit the ball hard very much lately. To be able to have a plan [in the fifth] and execute a plan personally was really nice for me. And then the boys kept tacking on after that. … That’s good baseball right there. I’m proud of the boys for doing that.”

Casali didn’t know he’d be filling in behind the plate until about 5 p.m., as the Giants planned to have Bailey catch before their starting backstop felt some soreness in his side after taking some early swings on Monday. Bailey, who was attempting to work out of his 5-for-66 (.076) slump, described the tightness as “very, very minor” and said it was more of a precautionary scratch, though he’s likely to sit out Tuesday’s game as well.

“It’s my job to be ready in any situation that calls on me, even if Patty gets scratched,” Casali said. “In a perfect world, I’m ready to go. I’m generally always fresh. I was ready to go today. They told me a little bit late, but it’s what we get paid to do.”

Matt Chapman tacked on another insurance run in the sixth, cranking a sweeper from Cannon out to left field for his team-leading 20th home run of the season. Chapman became only the second Giants third baseman to tally at least 30 doubles, 20 homers and 10 steals in a single season, after Hall of Famer Freddie Lindstrom in 1930.

The Giants collected 12 hits and finished 4-for-6 with runners in scoring position en route to scoring five runs for only the second time since Aug. 9. The offensive turnaround was welcome news for left-hander Kyle Harrison, who earned the win after firing six innings of one-run ball.

Harrison fell into trouble after the White Sox loaded the bases with no outs in the third, but he struck out Lenyn Sosa swinging on three pitches and then got out of the jam with the help of Estrada, who made a diving stop on Luis Robert Jr.’s 107 mph grounder and then threw to Fitzgerald to start an inning-ending double play.

“Hell of a play by Estrada over there,” Harrison said. “I wanted to challenge [Robert] with a heater. I wanted to get ahead. The guy was ready for it. He put a good swing on it, but it was an even better play. Nice to have ‘T’ back.”

The only damage against Harrison came in the fifth, when No. 9 hitter Brooks Baldwin walked, stole second and scored on Robert’s RBI single to right field. By completing six frames, Harrison reached 117 innings on the season, eclipsing his previous career high of 113, which he set in the Minors in 2022.

The White Sox scored twice in the eighth to pull within two and then threatened in the ninth after putting a pair of runners on with one out against Jordan Hicks, who was brought in to close with Ryan Walker unavailable. Casali once again came through with some big blocks to help a slightly overamped Hicks get through the jam and pick up his first save for the Giants.

“That was fun,” Hicks said. “Stressful, but fun.”