Happ knew he had big shoes to fill replacing Oswalt, who left Houston one victory behind Joe Niekro for the most wins in club history. And early on, it looked as if his debut would be a struggle like Oswalt's first start for the Phillies, which was going on simultaneously.
Happ allowed a leadoff double to second baseman Rickie Weeks and then walked right fielder Corey Hart. It wasn't exactly the first impression Happ was looking for in front of his new teammates and fans.
"I'm sure there was a lot of anxiety and nerves throwing for the first part of it," manager Brad Mills said.
But Happ overcame those nerves and recovered against the heart of the Brewers order. He struck out left fielder Ryan Braun, got first baseman Prince Fielder to fly out and got Casey McGehee to ground out.
"To get through the first inning and get a few big outs gives you confidence as the game goes," Happ said. "I started to settle in a bit, felt better and made good pitches."
Happ didn't make many bad pitches the rest of his start and showed why the Astros coveted him in the deal. He didn't give up another hit until the sixth when McGehee singled.
But those two hits were all Happ surrendered. He left after the sixth and finished with six strikeouts to become the first Astros pitcher since Roger Clemens in 2004 to toss six or more scoreless innings in his debut.
"If that's what we're getting, we got a pretty good pitcher," third baseman Chris Johnson said. "He was real good tonight. ... He was pretty nasty."
The Brewers are all too familiar with Happ's ability as a pitcher.
"We faced him last year right before Cliff Lee," Hart said a few hours before the game, "and most of us thought [Happ] was better."
With Happ cruising on the mound in his debut, the Astros gave the left-hander a welcoming gift that they hadn't granted Oswalt too many times this season: ample run support.
Playing without Lance Berkman and right fielder Hunter Pence, who had the day off, the Astros lineup gave Happ a chance to get into a groove. Right fielder Jason Michaels homered in the second to give them a 1-0 lead.
Two innings later, Michaels worked a two-out walk and first baseman Pedro Feliz singled. Then, Johnson jacked a homer to right field to expand the Astros' lead to 4-0.
"When we're up there, we're thinking about how we can get on base and keep the inning alive," Johnson said. "That's what we were able to do that inning."
And that was more than enough to make Happ's debut a successful one.