SAN FRANCISCO — Eight pitches into his start, Cincinnati's Johnny Cueto was in pain, and the Reds' playoff hopes were hurting, too.

Cueto, the Reds' ace and a 19-game winner in 2012, was out of the game practically before anyone knew what had happened. It was a huge gift to the Giants, one that should have tilted the contest, and perhaps the entire National League Division Series. They failed to take advantage, losing 5-2 to the inspired Reds to dig a deep hole for themselves as they try to resurrect the magic of 2010.

After eight pitches, in the middle of an at-bat by Marco Scutaro, Cueto began pacing around the mound area in obvious pain. Cincinnati manager Dusty Baker came out to investigate, and his starter trudged to the dugout, to the wonderment of Giants fans.

Afterward, Baker explained that Cueto had felt the first twinge while warming up before the game. He made it worse when he stopped his delivery midstream while pitching to leadoff hitter Angel Pagan, who had stepped out of the batter's box. And it really flared up during Scutaro's at-bat, sending Cueto to the showers.

"That was a little bit of a curveball, for both teams," Giants right fielder Hunter Pence said. "It's unfortunate that Cueto got hurt. He's a great pitcher. ... I hope he's all right."

The Reds' entire postseason pitching strategy might have gone into spasm at that moment, but the team adjusted smoothly on the fly. If the Cincinnati relief pitching was heroic Saturday, no one should have been surprised. This was the best bullpen in the major leagues this year, with 56 saves, an ERA of 2.65 and an opponents' batting average of .219.

Long reliever Sam LeCure finished the first inning and threw the second, buying time for Mat Latos — who had been scheduled to pitch Game 3 in Cincinnati. It was the first relief appearance of Latos' career, and he went four solid innings, giving up a single run.

"You got to give big props to Latos," Baker said. "Here is a guy sitting back, relaxed, thinking he was going to pitch next week at home and now all of the sudden, boom, this is his first playoff game. It was a great feat by him."

Sean Marshall, Jonathan Broxton and flame-throwing closer Aroldis Chapman finished the work for the Reds.

Chapman made it interesting, though, surrendering a single to Joaquin Arias and walks to Xavier Nady and Marco Scutaro, and allowing a run on a wild pitch. But he struck out slugger Buster Posey on a high 100-mile-an-hour fastball to end the game.

Giants starter Matt Cain was 0-2 with a 5.54 ERA against the Reds during the regular season (16-3 against the rest of MLB), and they pounded a pair of home runs early Saturday — a two-run shot to left field by Brandon Phillips in the third inning and a solo blast to right by Jay Bruce in the fourth. Bruce entered the game batting .462 against Cain in 13 previous at-bats and went 2 for 2; he also rapped a second-inning double.

"The pitch to Bruce wasn't too terrible, but the hanging breaking ball to Phillips is just something you don't want to happen in definitely a big-game situation like this," Cain said. "The hanging breaking balls in these games, they always hurt a little bit more."

Phillips' homer plated the first runs ever against Cain in the postseason. He had started his career with 23? consecutive scoring innings (mostly during the 2010 playoffs and World Series), the fifth-longest streak in MLB history.

Cain was gone after five innings, lifted for pinch hitter Aubrey Huff to start the bottom of that inning.

The Reds added two insurance runs against Santigo Casilla in the top of the ninth, on consecutive singles by Ryan Hanigan, pinch hitter Xavier Paul and Phillips, followed by a wild pitch by Casilla and a passed ball allowed by Posey.

No surprise that it was Posey, the National League batting champion and MVP candidate, who finally broke the ice for San Francisco. He greeted Latos' first pitch of the sixth inning and launched it into the left-field seats to cut the score to 3-1.

The Giants threatened again in the eighth inning, but Gregor Blanco, who had two hits in the game, struck out looking on a pitch he felt was low and away.

It wasn't enough offense to beat Cincinnati on this night. The question is whether the Giants, who won the openers of all three postseason series in 2010, can bounce back from this disappointment.

"We have to leave it behind," Blanco said. "We still have more games to play. This was just the first one. It was really important for us if we would win it, but we'll take the loss, and tomorrow's always another day.

You can reach Staff Writer Phil Barber at 521-5263 or phil.barber@pressdemocrat.com.