HOUSTON -- With a return trip to the postseason increasingly unrealistic, San Diego can still use the remainder of 2023 to evaluate players. In Sunday’s 12-2 loss to the first-place Astros at Minute Maid Park, the club’s hope is that many younger Padres will learn from the experience.

Veteran left-hander Rich Hill had a 10.71 ERA over his previous six appearances, so manager Bob Melvin skipped Hill’s turn in the rotation while giving young right-hander Matt Waldron his third career start on Sunday. It began well with two shutout innings, but things unraveled in the third and fourth, when the Astros scored four times.

Jose Altuve’s two-run home run did the most damage.

“With Altuve, you never, ever want to throw a fastball right there to him,” Waldron said. “He's an All-Star at that. I gave him what he was looking for, and he didn't miss it.”

With the series-clinching win, Houston (82-62) moved to a 2 1/2-game lead over Seattle (79-64) in the American League West standings. San Diego (67-77), which was without two key veterans in Fernando Tatis Jr. (planned rest) and Manny Machado (right elbow soreness), dropped to double-digit games below .500 for the fourth time this season.

“It feels bad,” All-Star outfielder Juan Soto, who was 0-for-2 with a walk, said of San Diego’s underwhelming season. “It feels the worst. We still have a chance, so we've got to go out there and keep playing baseball and figure it out. Things aren't going our way.”

In five innings, Waldron (0-3, 5.55 ERA) allowed four runs on four hits and two walks. After an error allowed Houston’s fourth run, Waldron regrouped and retired six straight to end his day.

“Today fell short of the standard I hold myself to,” he said. “Any starter would not be happy with this. But I'm learning. It usually comes down to a couple of mistakes, and you have to minimize those. Those last six outs, I felt pretty good and I put some good sequences together.”

Once Waldron exited, things quickly spiraled. Six straight Astros got hits off Tim Hill to start what became an eight-run sixth inning, one which featured two RBI triples by Kyle Tucker. After being skipped in the rotation, Rich Hill finished things out in mop-up duty.

“Waldron tried to hold them for five innings, and he was pretty tough,” Soto said. “Their offense is really good, and they broke through. You have to tip your cap.”

Among position players, results were a mixed bag. Second baseman and leadoff hitter Ha-Seong Kim, who is in his third Major League season and second as a full-time starter, picked up another knock and made a spectacular third-inning stop to deny a hit to Jeremy Peña. Kim is hitting .271 with 17 home runs and a .778 OPS.

“From defense to offense to baserunning, everything he does would suggest this is the start of something really good, and [he’s] a guy that I consider is on the verge of being an All-Star,” Melvin said of Kim. “In the years to come, I expect him to be.”

The outlook is less clear for young catcher Luis Campusano and rookie third baseman Eguy Rosario, who made his first career start.

With Waldron’s propensity for knuckleballs, the Astros ran aggressively and stole four bases off Campusano. He also had two passed balls and two errors, including a drop on a potential forceout at home during Houston’s sixth-inning onslaught. Campusano is now San Diego’s primary catcher after Gary Sánchez’s season-ending wrist fracture.

At the plate, Campusano didn’t help matters with a bases-loaded popout in the first, which was San Diego’s best opportunity to take a lead vs. Astros starter J.P. France. Matt Carpenter followed with a strikeout, and the window shut.

“We left some runners on early when it could have been impactful, and then things got away from us,” said Melvin, whose team committed four errors. “Not our finest moment.”

As for Rosario, the rookie may have showed some nerves when he misplayed José Abreu’s fourth-inning grounder, allowing another run to score. He made amends, however, with his first career homer in the seventh. The 409-foot blast by Rosario -- ranked as San Diego’s No. 14 prospect by MLB Pipeline -- would have been a homer in all 30 Major League ballparks, according to Statcast.

“You like to see a guy respond like that,” Melvin said.

“He's been good through the Minor Leagues, and we know what kind of player he is,” Soto said. “An error won't mean anything. Everybody has them. We know what he can bring to the team, so I'm happy for him that he got his first [homer].”