Down 8 1/2 games in the National League wild-card race after walking off the field following a 5-3 win, the Giants nonetheless will continue to pursue their postseason dream when they complete a three-game home series against the Texas Rangers on Sunday.
The decision on Posey came hours after the Giants suffered a back-breaking, 7-6 loss to the Rangers on Friday night in which Posey played the entire 10 innings, watching Texas deny San Francisco a much-needed win with a two-out, two-run homer in the ninth after a game-extending error.
Posey's surgery Monday will be performed in order to repair a torn labrum and remove bone spurs from his painful right hip.
The 31-year-old made himself available Saturday but saw no action in another exciting finish that saw the Giants' Mark Melancon strand the bases loaded in the ninth after the Giants had blown most of their 5-0 lead the inning before.
The win was just the Giants' third in their last 10 games.
"You don't want to say you're out of it until you're out of it," Posey said. "But, unfortunately, where we are, it makes sense to take care of it right now."
Posey said having the surgery basically one month before season's end will give him an additional month in the offseason to rehab. He not only plans to be with the Giants for spring training in February but continue to be the primary catcher.
Nick Hundley contributed a hit, a run and two walks to Saturday's win as Posey's replacement. The start was Hundley's 46th of the season.
Hundley and his teammates will go up against Rangers right-hander Yovani Gallardo (7-2, 6.06) in the series finale.
The 32-year-old has made 10 previous starts against San Francisco, going 6-4 with a 3.21 ERA.
Giants outfielder Andrew McCutchen, who has led off the first inning with a home run and a double in the first two games of the series, has enjoyed a big-numbers career against Gallardo, going 19-for-55 (.345) with three doubles, two triples and four homers.
San Francisco will hand the ball to left-hander Derek Holland (6-8, 3.75), who has made 158 of his 208 career starts for the Rangers.
Since leaving Texas after eight seasons for the Chicago White Sox in free agency in 2017, the 31-year-old has faced his former employer twice and gotten bombed both times, running up a 13.50 ERA while going 0-2.
Old pal Rougned Odor has been especially rough on Holland, delivering a single and a three-run homer in four at-bats.
Odor has homered twice in the series, both times in critical situations. It was his game-tying homer in the ninth inning that extended the series opener and led to a 7-6 win in 10 innings. Then on Saturday, he blasted a three-run shot to rally the Rangers within 5-3 in the eighth inning, setting up the tight finish.