Brandon Bielak won’t be able to remember what it feels like taking the mound in front of thousands of fans in his first start. He won’t recall looking up into the stands and seeing his family swelling with pride. And Bielak won’t even be able to remember celebrating a win with his teammates.

Despite five scoreless innings from Bielak in the first start of the 24-year-old’s Major League career -- and despite a two-run, go-ahead homer by Alex Bregman in the eighth inning -- the Astros lost to the D-backs, 5-4, on Thursday night at Chase Field.

• Box score

“We had some positive things happen tonight,” Astros manager Dusty Baker said. “Young Bielak threw the ball great.”

Bregman’s homer put the Astros ahead, 4-3, but All-Star reliever Ryan Pressly couldn’t hold the lead in the ninth. All four batters reached against Pressly, with Kole Calhoun hitting a two-run double to right field for the walk-off win.

“It’s devastating,” Baker said. “But we’ve got to put that behind us. It hurts today. It hurts a lot. Pressly hadn’t had much work. He was champing at the bit to get some work, and we had it all lined up. It was the perfect situation. The Diamondbacks mess up our plans.”

Bielak, who earned wins in his first two relief outings before moving into the rotation to replace the struggling Josh James, gave up two singles and three walks but didn’t allow a runner to reach third base in five innings. He threw 82 pitches (48 strikes) and was admittedly gassed at the end.

“He came off the mound and he was thoroughly exhausted,” Baker said. “We stretched him as long as we could. We asked him, ‘Hey man, are you out of gas?’ And he was wobbling to sit down. That’s the longest he’s gone [this year]. And so, no, we were satisfied with what he gave us tonight.”

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Bielak’s fastball, which he threw 54 percent of the time, was sitting between 92 to 94 mph. He threw his circle changeup in some big spots, too, and wasn’t easily rattled. If he lost command of the strike zone, he was able to calm himself down and get back into it.

“I thought I felt pretty good out there this afternoon,” he said. “[Catcher Martín Maldonado] called a great game out there. I did get a little tired in the fifth.”

Bielak is one of nine Astros pitchers who have made their Major League debuts this year. A combination of injuries to key pitchers like Justin Verlander, Jose Urquidy, Brad Peacock, Austin Pruitt, Chris Devenski and Roberto Osuna thinned the Houston pitching corps in a season when rosters were expanded because of the coronavirus pandemic.

Bielak, Houston's No. 13 prospect per MLB Pipeline, made his debut July 27 against Seattle and allowed one earned run in 3 1/3 innings. He followed that up four days later with a scoreless two-inning outing against the Angels. Bielak threw a total of 93 pitches in those outings.

“I can’t really use the word ‘comfortable,’” Bielak said. “Just hopefully I’ll be consistent every outing. I’m just going out there and doing what I can do and staying within myself. That’s the main goal.”