SAN FRANCISCO -- Right-hander Taijuan Walker's wildness sent San Francisco Giants star Buster Posey to the trainer's room Monday afternoon.

Then it sent the Arizona Diamondbacks to defeat.

The Giants took advantage of pitcher Matt Moore's swinging bunt to score three runs in a bizarre sequence that led directly to a 4-1 victory over the Diamondbacks in the home opener at AT&T Park.

Moore teamed with new closer Mark Melancon to cool the hot-hitting Diamondbacks, who began the series with a five-game winning streak and led the major leagues in several offensive categories, including runs (48).

"It was nice to have a shot at redemption," said Moore, who was beaten by the Diamondbacks in Arizona last week. "I didn't get the job done, let the game slip away. It was nice to come back and get another shot at it."

The Giants won despite losing Posey in the first inning when the former National League Most Valuable Player was hit in the head by a Walker fastball.

Posey exited the game despite getting to his feet within a couple of minutes and never losing consciousness.

Afterward, Giants manager Bruce Bochy reported good news.

"He's fine," Bochy said, claiming he would have considered keeping Posey in the lineup if he weren't the catcher. "It's a scary moment, a dangerous moment. But that doesn't mean there's something there."

Bochy said Posey would be monitored overnight and re-examined in the morning. It is possible Posey could be back in the starting lineup for the second game of the series Tuesday night.

The teams battled to a scoreless tie until Brandon Crawford led off the Giants' fourth inning with a double.

After an out and two walks, Moore came to the plate with the bases loaded and fouled off a safety-squeeze bunt attempt.

"Just me knowing my limitations," said Moore, whose primary goal was to avoid an inning-ending double play.

The left-handed swinger then took a full cut at the next pitch, but the ball went no farther than the average bunt, about 30 feet between the mound and first base.

Walker got to it quickly, but in his haste to force Crawford at the plate, he first bobbled the ball and then threw low and wide of catcher Jeff Mathis, allowing the first run to score.

"I was a little bit shaken up after hitting Posey in the head," Walker said of his outing in general and the poor throw to the plate in particular. "It's always a little scary."

Mathis chased the ball to the backstop and threw back to Walker, who was covering home plate, in a bid to get Joe Panik, who also was attempting to score. Mathis' throw got away from Walker, allowing not only Panik to score but also Jarrett Parker, who had begun the play at first base.

Walker and Mathis were both charged with errors on the play, the only errors of the game by either team.

"We call them RTIs -- runs thrown in. We had three of them," Bochy said. "He hit it in the right spot. Some rushed throws and we took advantage of it."

The Diamondbacks (6-2) closed within 3-1 on a solo home run by Yasmany Tomas leading off the fifth, but that was the extent of the damage against Moore (1-1), who allowed only three hits in eight innings. He struck out five and did not walk a batter.

Melancon, who blew a save in his Giants debut at Arizona on Opening Night, posted his second save with a scoreless ninth that included a leadoff double by Chris Owings off the glove of San Francisco third baseman Eduardo Nunez.

The Giants (3-5) added an insurance run off Diamondbacks reliever Randall Delgado in the seventh on a sacrifice fly by Crawford. Hunter Pence, who had led off the inning with a double, scored on the play.

Walker (1-1), who had beaten the Giants last week, took the loss, allowing three runs (two earned) and five hits in five innings. He walked four and struck out two.

"You saw what happened with Buster," Diamondbacks manager Torey Lovullo said of Walker. "He lost his focus there for a moment."

Pence had two of the Giants' six hits.

Mathis doubled for the Diamondbacks, who had totaled 77 hits in their first seven games.

NOTES: The fastball that hit Giants C Buster Posey clocked in at 94 mph. ... RHP Taijuan Walker hit 17 batters while pitching for the Seattle Mariners the past two seasons, tying him for sixth most in the American League. ... Giants LHP Matt Moore had batted only three previous times in his career with the bases loaded. He had struck out twice and gone 0-for-3 in those situations. ... Diamondbacks LF Yasmany Tomas didn't hit his first home run last season until Arizona's 10th game. He went onto hit a career-best 31. ... The Giants improved to 14-4 in home openers in the 18-year history of AT&T Park. ... The Diamondbacks were attempting to open a season 7-1 for the first time in franchise history .