Bud Norris was pitching well until he felt something in his cranky left knee. It kept getting worse, forcing an early departure.

"It's sore," Norris said Tuesday night after lasting 3 1-3 innings in the Houston Astros' 6-3 loss to the San Francisco Giants. "I tweaked the knee, no doubt about it. Ever since Colorado (on May 31) I've tried to get better. I'm pretty frustrated about it."

Norris (5-4) lost his third straight start and is winless over his past four decisions.

"He had his good stuff too," Astros catcher Jason Castro said. "He was able to locate. I know he landed hard on it and I'm sure that didn't feel too good."

Madison Bumgarner had a season-high 12 strikeouts and hit his first major league home run to lead the Giants.

Brian Bixler singled, doubled and drove in both runs for Houston, which has lost 12 of its last 16 games.

Bumgarner (8-4) struck out the side in the first, allowed an unearned run in the third following Brandon Crawford's two-out throwing error then kept the Astros in check before leaving in the eighth.

The left-hander reached double digits in strikeouts for the third time in his last five starts, didn't walk a batter for the third time this season and retired 11 of 13 until pinch-hitter Brian Bogusevic's single leading off the eighth.

Two batters later Bixler hit an RBI double to pull Houston within 4-2 and chase Bumgarner.

Bogusevic has four hits in eight pinch-hit appearances this season and is hitting .340 (17-for-50) over his career as a pinch-hitter. Bixler has four hits in 12 at-bats over his past three games.

Norris, who walked three and hit a batter, appeared to get hurt after chasing a pop-up in foul territory in the fourth. Norris was unable to make the catch and nearly collided with Gregor Blanco, the on-deck hitter, then slowly walked back to the mound where he met with a team trainer and manager Brad Mills before attempting a few warm-up pitches.

"He started to change his motion a little bit so that's why we had to go get him," Mills said. "He was throwing the ball well and then those three walks in one inning."

Reliever David Carpenter replaced Norris and pitched 2 2-3 scoreless innings but the Astros couldn't get much going offensively against Bumgarner and fell to 2-2 on their nine-game road trip.

Notes: Giants manager Bruce Bochy said Aubrey Huff will likely DH when the team travels to Seattle for another round of interleague play Friday. ... Bochy met with general manager Brian Sabean, pitching coach Dave Righetti and bullpen coach Mark Gardner to discuss struggling RHP Tim Lincecum. They discussed possibly having Lincecum skip a start or move to the bullpen but decided to keep him in the rotation to work out his problems. Lincecum (2-7) is winless over his previous eight starts. ... Houston 1B Carlos Lee is anxious to begin his rehab assignment but the Astros are being cautious with him while he recovers from a strained left hamstring. ... San Francisco signed 22 of its 40 draft picks on Tuesday, including first-rounder Chris Stratton. ... The Astros signed five of their draft picks. ... RHP Matt Cain (7-2) chases his seventh consecutive win on Wednesday against Houston LHP J.A. Happ (4-6). ... Rory McIlroy, in town to defend his U.S. Open championship, threw out the ceremonial first pitch.