SAN FRANCISCO -- The next-man-up mantra has been woven into the DNA of the 2021 Giants.

After Buster Posey was scratched from the lineup with back tightness on Friday night, his replacement, Curt Casali, stepped in and homered to help seal a close win for the Giants.

“I think these players pride themselves on stepping up for one another,” manager Gabe Kapler said. “We talk a lot about being a good teammate. That's really what it's all about. 'OK, you're not ready, you're not available. I'm ready at the drop of a hat for you.' It's kind of that mentality. I think Curt yesterday, by way of example, wasn't just excited about getting an opportunity to play. He was excited about protecting and being there for Buster. I think you can probably look around our team and see a lot of that over the course of the season.”

Kapler didn’t have to wait long for another example of that trend to emerge. LaMonte Wade Jr., who has been filling in at first base in place of the injured Brandon Belt, went deep and Casali capped his second consecutive start behind the plate with a walk-off double that lifted the Giants to a 6-5 comeback win over the A’s in 10 innings on Saturday night at Oracle Park.

The Giants entered the bottom of the 10th trailing, 5-4, but Steven Duggar knocked in Brandon Crawford with a single up the middle to tie the game and then scored from first on Casali’s double down the left-field line to give San Francisco its Major League-best 50th win of the season. It was the second career walk-off hit for Casali, who struck out in his first four at-bats and battled his own bout of back tightness throughout the night.

“I don’t think he was at his best there in his last at-bat,” Kapler said. “He just got especially gritty and tough. It was a great send by [third-base coach Ron Wotus], great jump across the board. That was a great play.”

Duggar, one of the fastest players on the team, flashed his game-changing speed as he sprinted around the bases, sliding in ahead of Elvis Andrus’ throw to the plate to seal the Giants’ first walk-off win of 2021.

“Just head down, high knees, let’s try to get home,” Duggar said, laughing. “I probably would have blown through a stop sign, too, if Wo would have thrown it up. It was quite a blur.”

After taking the first two games of the Bay Bridge Series, the Giants (50-26) have now won 10 of their last 11 games to move 24 games over .500 for the first time since July 10, 2016. San Francisco has either won or split its last 10 series dating to May 25, tied with the Yankees for the longest such streak this year.

“It’s really special to be a part of,” Casali said. “Fifty wins is more than a lot of people thought we’d get at this point in time, and we have plans to keep on going.”

The Giants haven’t missed a beat thus far thanks to contributions from depth pieces like Casali and Wade, who has flourished since he was acquired from the Twins in exchange for right-hander Shaun Anderson in February. Wade began his week with a surprise detour to Triple-A Sacramento after he was optioned to clear a pair of roster spots for Darin Ruf and Alex Dickerson on Monday, but the demotion proved brief, as he was recalled on Friday after Belt landed on the injured list with a right knee injury.

With right-hander Frankie Montas on the mound for the A’s, Kapler opted to slot the left-handed-hitting Wade back into the leadoff spot and start him at first base on Saturday. Wade responded by going 2-for-4 with a walk, with each of his four batted balls registering exit velocities over 100 mph.

Wade briefly put the Giants ahead, 2-1, after bashing a two-out, two-run blast in the fifth inning. The 27-year-old fell behind, 0-2, but he fouled off three consecutive pitches from Montas before smoking a misplaced 97.8 mph fastball out to right-center field to erase a one-run deficit and collect his career-high fifth home run of the year.

“He just has that look in his eye,” said left-hander Alex Wood, who gave up two runs (one earned) over 5 1/3 innings. “He’s super, super focused. Obviously, his swing feels good at the plate. Watching him go up there and compete every pitch, every AB, from the first inning to the ninth inning, it’s just been great. I’m really happy for all the success that he’s had.”

The A’s opened up a 4-2 lead behind Matt Chapman’s two-run shot in the seventh, but the Giants pulled within one after Wade drew a two-out walk and scored on Wilmer Flores’ RBI single to left field in the bottom of the inning. Donovan Solano then delivered a game-tying, no-doubt shot to left field in the eighth.

Wade ended up setting the table for another Giants rally after smacking a leadoff single to the opposite field off Lou Trivino in the bottom of the ninth. He advanced to second on Ruf’s two-out walk to put the game-winning run in scoring position for Crawford, who came through with a single to shallow left field. Wade was waved home by Wotus, but he was easily thrown out after Tony Kemp and Chapman executed a perfect relay throw to catcher Sean Murphy, sending the game into extra innings.

Still, the Giants ultimately wouldn’t be denied, with Casali sending their fans home happy with his game-winning hit in the 10th.

“Walk-off wins are probably one of the best things in baseball,” Casali said. “To see the excitement and the joy on everybody’s face after we won the game was pretty incredible. ... We’re rolling right now, and it feels really good.”