Oswalt was forced from his start on Saturday with a strained left oblique muscle, and he may have to miss at least one start, depending on how he feels in the next day or two.
"Last night, I felt like I had good enough stuff to throw the whole game," he said prior to Sunday's finale at PETCO Park. "The last four or five starts have been the best stuff I've had since coming up [to the Major Leagues in 2001]. The ball's coming out of my hand great.
"I had problems last year, the ball coming back over the plate as far throwing down and away. But now I'm staying through, and I'm able to throw down and away when I want to."
Oswalt likely will throw lightly on flat ground on Tuesday, at which time he'll know more about the extent of the injury. But as of Sunday, the outlook was rather grim.
"It doesn't feel great," Oswalt said. "It hurts. It hurts bad."
Carlos Lee, who also left Saturday's game prematurely, was not in the lineup on Sunday. The left fielder strained a hip flexor muscle while sliding into second base.
Lee will be evaluated on a day-to-day basis, but he's hopeful he'll be able to play Monday.
"I'm going to give it a shot," Lee said. "I'm going to get treatment and go out and run. I can swing. It doesn't bother me to swing at all. I don't want to be out there if I'm not close to 100 percent. You put the team in position where they're playing short. I'll do my best for tomorrow, and hopefully I'll feel good."
Lee pinch-hit on Sunday, popping to third base in the ninth inning.
Cleanup: Mike Lamb joked that he was "salivating" after Lee left the game on Saturday, as he feigned excitement over the opportunity this may create for him.
In reality, of course, Lamb was not happy that his teammate will be out of commission for a day or two. But Lamb did manage to look on the bright side.
"I'm trying to regain my title as the only bench player in baseball that bats fourth when he plays," Lamb said. "That's my goal today."
Mission accomplished. Lamb played first base on Sunday, batting cleanup.
Gutierrez debuts: Juan Gutierrez flashed a thousand-watt smile following Sunday's game, not long after he made his Major League debut with a relief appearance in the eighth inning.
Gutierrez struck out his first batter, Rob Mackowiak, and recorded two more quick outs to end his 13-pitch debut.
"[On Saturday], the first time the [bullpen] phone rang and [manager Phil Garner] said, 'Gutierrez,' my heart started to pound a little bit hard," Gutierrez said. "Today, it was normal. I didn't put any pressure on myself, like I said the first day that I got here."
Gutierrez kept the ball from his debut as a memento.
Pomp and circumstance: The Astros will retire Jeff Bagwell's uniform No. 5 prior to the Aug. 26 game with the Pirates during a pregame ceremony that is expected to begin at approximately 12:20 p.m. CT. Limited tickets are available.
Craig Biggio and Brad Ausmus are scheduled to speak at the ceremony, along with club owner Drayton McLane. All living players whose numbers have been retired by the Astros also are expected to be in attendance, including Larry Dierker, Jimmy Wynn, Nolan Ryan, Jose Cruz and Mike Scott.
Bagwell's daughters, Bryce, 6, and Blake, 4, will throw out ceremonial first pitches to their dad.
Coming up: The Astros will return home on Monday to begin a 10-game homestand, during which they'll host three teams. First up are the Nationals, who will be in town for four games. Monday's opener will feature right-hander Jason Jennings (2-8, 6.16 ERA) and former Astro Tim Redding (2-3, 2.88 ERA).
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.