Posey will be re-evaluated Tuesday but, "He's doing good, he's doing fine," manager Bruce Bochy said. Posey is likely to sit out Tuesday's game.
Taijuan Walker's 0-1 fastball with two out in the first inning sent the Gold Glove catcher immediately to ground. Athletic trainer Dave Groeschner sprinted toward the plate and Bochy was right behind. Posey got up on his own but exited the game, with Nick Hundley entering to pinch run and stay in the game as catcher.
Bochy said if Posey weren't a catcher, he might have been OK to stay in the game, but the Giants weren't taking a chance with the 2012 NL MVP and 2010 NL Rookie of the Year.
"It's a scary moment, dangerous moment," Bochy said. "It's one of the worst sounds you can hear in baseball, the ball hitting the helmet. It's a scary moment. There's been a lot of damage to hitters hit in the head."
Walker (1-1) was booed at the conclusion of the inning, when he came up to bat and at every other opportunity by the AT&T Park sellout crowd of 42,129. Home run king Barry Bonds was among those in attendance.
Matt Moore (1-1) struck out five in eight impressive innings.
The Giants scored three runs in the fourth on a trio of successive plays at the plate. Brandon Crawford doubled, then one out later Joe Panik and Jarrett Parker drew consecutive walks. Walker fielded Moore's chopper midway up the first-base line and fired wildly to home, allowing Crawford and Panik to score. Catcher Jeff Mathis retrieved the ball and also made an errant toss well past the plate. First baseman Paul Goldschmidt got the ball in nearly the same place where Walker had been, and his throw back home was too late as Parker slid across safely. Two errors were charged.
Crawford added a sacrifice fly in the seventh after Hunter Pence doubled to start the inning.
Yasmany Tomas led off the fifth with his first home run of the season for Arizona.
Moore, beginning his first full season with the club after being acquired from Tampa Bay at the 2016 trade deadline, took the loss after working 5 1/3 innings last Wednesday at Arizona.
He got through a 1-2-3 first on seven pitches and was on a roll. He gave up three hits and didn't walk a batter before new $62 million closer Mark Melancon finished for his second save in as many days and first at home after blowing a save opening day in Phoenix.
Moore plunked David Peralta on the upper right arm to start the eighth.
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