"I'm pretty frustrated about it," Norris said. "I'm trying to go out there and help these guys as much as I can because I'm a competitor and want to compete, but I'm frustrated and it's pretty sore now and I have to come back tomorrow and see how it feels."
After going 4-0 with a 0.35 ERA in his first four starts in May, Norris (5-4) is 0-3 with a 10.91 ERA in his last four starts, two of which have been cut short because of injuries. He lasted only 1 2/3 innings on May 31 in Colorado because of a sore hip flexor.
"Ever since Colorado, I've been going out there and trying to get better and the hip flexor is feeling a lot better," he said. "But I definitely tweaked the knee tonight and we've got to see how it feels tomorrow."
Giants starter Madison Bumgarner (8-4) struck out a season-high 12 batters and didn't walk a batter in 7 2/3 innings to improve to 13-2 in his last 16 home starts. He also clubbed his first career homer to snap the Giants' 16-game homerless drought at AT&T Park.
"He's been pitching great here, he has all year, and he supplied some power," Giants manager Bruce Bochy said. "He just pitched terrific. He had great command, [had] all his pitches. He hit both sides of the plate. He pounded the strike zone, and when you do that you'll throw efficiently."
Norris appeared to suffer his knee injury when he sprinted to first base to field a grounder off the bat of Melky Cabrera and stepped on the base for the final out of the first inning. He aggravated it racing to field a popup in foul territory in the fourth inning after allowing three walks and a two-run single in the inning.
"When you have something like that that's nagging, it's definitely on your mind and it's hard to pitch through for sure," Astros catcher Jason Castro said. "I'm sure it was affecting him when he was landing, so that might have been part of what was going on."
The Astros took advantage of a two-out throwing error by shortstop Brandon Crawford to steal an unearned run in the third. Jose Altuve reached second base on the error, stole third base and scored on a two-out single by Brian Bixler. Bumgarner led off the bottom of the inning with a homer to tie the game.
"It was just kind of right on his bat," Castro said. "He put a good swing on it, he swung hard and he's an athletic kid, so he's definitely capable of that. Any time you elevate the ball, anyone can hit it out. It's tough to give that up, but at the same time, you've got to tip your cap to him. He did a great job."
Norris had allowed two hits and one run before walking three of the first four batters he faced in the fourth inning, loading the bases. Crawford followed with a two-run single that nearly hit Norris' glove before scooting past a diving Jed Lowrie and into center field to put the Giants ahead, 3-1.
"He was throwing the ball so well," Astros manager Brad Mills said. "You get the three walks in the one inning, obviously hurt, and two of them scored that inning and one of the walks scored later. The situation with Bud is when you start changing your mechanics because you're afraid you're going to hurt something you have issues."
Norris left the game moments later with the knee injury, which he suffered chasing down a Bumgarner pop fly in foul territory that Castro lost off the bat. Norris threw a few warmup pitches for Mills and athletic trainer Rex Jones before leaving the game.
"I took a good sprint at it, gave it a real good effort, but those last five steps pounding on it definitely didn't help it," he said. "I threw those extra couple warmup pitches and I appreciate playing on the side of precaution, but I'm pretty frustrated."
Relief pitcher David Carpenter did a nice job for the Astros, throwing 2 2/3 scoreless innings, but Cabrera's RBI single in the seventh off Brandon Lyon stretched the lead to 4-1. Brandon Belt hit a two-run homer in the eighth off Wesley Wright to send the Giants on their way to a win.
"We fought back into it and gave ourselves a chance, and I think we do that every night," Astros first baseman Matt Downs said.