It was enough that Sampson, making a fill-in start, pitched as well as he has in relief.
Sampson (4-3) has posted a 2.16 ERA in 13 relief appearances since his last start on May 25. He needed only 55 pitches to last five innings.
"Sampson was outstanding," said Astros manager Cecil Cooper. "My usual word for him is very efficient, and he was. ... He really gutted through five innings."
"[Sampson] hit his spots," said Atlanta's Chipper Jones. "He hit his spots well. He moved the ball in and out, changed speeds. For us to get [four] baserunners tonight is pathetic. Against anybody."
Sampson gave up two hits with no walks and two strikeouts.
"I really didn't even pace myself," Sampson said. "I pitched like I was coming out of the bullpen. ... I used all my pitches. I didn't save any pitches. I didn't worry at all. If I only got three innings, then I just got three innings."
Oswalt, who returned to Houston to be examined on Saturday after experiencing more pain in his left hip, is expected to rejoin the team Sunday. When asked if Oswalt might make another start before the All-Star break, Cooper said, "We're always hoping."
Cooper made out a new-look lineup that included Ty Wigginton batting leadoff for the first time this season.
Before the game, Cooper said "I like my chances" because of the strong on-base percentages of Wigginton (.456) and Loretta (.438) against left-handed pitchers. The two played key roles against Braves left-hander Jo-Jo Reyes as the Astros scored four times in the first two innings.
Loretta reached base in each of his three plate appearances against Reyes. He walked in the first and scored on a single to right by Carlos Lee, who then scored from second on Miguel Tejada's single to left.
Loretta's two-run double to right in the second drove in Wigginton and Brad Ausmus. Loretta added another double in the fifth but was thrown out at the plate by first baseman Mark Teixeira, after Brian McCann threw to first attempting to catch Lee.
"I had a little bit of luck tonight," Loretta said, after matching his career high for doubles in a game.
Even with Lance Berkman going 0-for-4 with a walk, the Astros had more than enough offense to beat the punchless Braves, who managed only four hits and have lost six of seven.
"Absolutely pathetic," said Jones, who was 0-for-4. "We didn't even get five hitters into our lineup and we're down 4-0. And then all of a sudden everybody's going, 'Oh no, here we go again.' And then guys start to press."
Added Jones: "If we keep playing one good game every five, it's going to be a long summer."
Reyes (3-7), who has not won in four starts since June 13, gave up six hits and four runs in 4 1/3 innings.
Sampson gave up his only run in the fifth on McCann's 15th homer, possibly earning another start.
"We'll kind of see how things go," Cooper said.
Said Sampson: "Obviously, my heart was in the rotation, but if the team needed me in the pen, I was going to go to the pen. I've gotten pretty comfortable out of the pen."
Byrdak pitched two scoreless innings behind Sampson, and Brocail and Valverde combined for two perfect innings.
Jason Perry, called up Friday when the Braves optioned right fielder Jeff Francoeur to Double-A Mississippi, was 0-for-3 with two strikeouts.
Braves LHP Mike Hampton threw five scoreless innings, allowing five hits with no walks, in an injury rehabilitation start for Class-A Myrtle Beach on Saturday. Hampton is returning from a strained pectoral muscle. ... Cooper started Reggie Abercrombie in center field but said Michael Bourn would start on Sunday after his second consecutive day off. ... Francoeur was 2-for-4 with a walk and an RBI in his first game with Mississippi on Friday.