Valdez was playing second base in place of Brandon Phillips, who was ejected in the eighth after arguing a check-swing call with home-plate umpire Andy Fletcher.
"It's part of baseball," Valdez said. "You're never going to be perfect. Anybody's going to miss a play. That was a bad moment that I made there because it cost us a baseball game. Tomorrow is a new game, and we'll see what happens."
The Astros rallied off reliever Sean Marshall, who entered the game after Homer Bailey delivered eight stellar innings.
Matt Downs, pinch-hitting in the ninth, doubled high off the wall in left with two outs. The ball was a foot or two away from clearing the fence for a walk-off homer. Marshall intentionally walked Matt Dominguez, then hit pinch-hitter Brandon Barnes with a pitch to load the bases.
Altuve hit the weak ground ball to Valdez, who had time to set up in front of it, but it dribbled between his legs.
"Your heart just sinks into your stomach when that happens," Baker said. "We have to stick by Wilson. No one feels worse than him."
Bailey did his part, allowing three hits and striking out nine.
The right-hander cruised through the first three innings, allowing just a single to Fernando Martinez in the second. He induced two groundouts to open the fourth before Justin Maxwell hit his 13th homer of the season, a line-drive shot to right field.
"The guy just went down there and got it and hit it out of right field," Bailey said. "Sometimes you have to tip your hat."
After hitting the next batter, Martinez, with a pitch, Bailey retired the next 10 in order. He struck out the side in the seventh.
With two runners on in the eighth, Bailey struck out Jimmy Paredes with a high fastball and got Tyler Greene to fly out to center to keep the score tied.
"Homer pitched great tonight," Baker said. "He got out of trouble at the end there. He reached back and got some more."
Marshall entered to start the ninth with Bailey at 107 pitches.
Bailey, from nearby La Grange, is 2-0 with a 0.60 ERA in four career starts at Minute Maid Park. His ERA in his last six starts was 7.03.
"Being home is so big," Bailey said. "Being able to be around family and friends and then having them watch you work is always fun."
The Reds plated one run or fewer for the first time since Aug. 7.
Zack Cozart singled to start the first inning and moved to second on Drew Stubbs' bloop single. Cozart went to third on a fielder's choice and scored on Ryan Ludwick's sacrifice fly, giving the Reds an early one-run lead.
The Reds tallied five hits over the next six innings off Lucas Harrell but couldn't score. Reliever Hector Ambriz replaced Harrell, who allowed seven hits and struck out three, after the seventh. Harrell recorded 13 groundouts and just two flyouts.
"I got a lot of ground balls, and that was sort of the plan," Harrell said.
"He broke about a dozen of our bats throwing that ball in on guys," Baker said.
Ambriz, Xavier Cedeno and Wilton Lopez held the Reds hitless over the final two innings.
Phillips, who'd entered the game with a 10-game hitting streak, finished 0-for-4 with two strikeouts.
Baker thinks his team would have had a chance if the game had gone into extra innings.
"It still hurts, because we were out of that inning," Baker said. "We had more pitchers than they had and more hitters than they had."
He did find a silver lining: Maybe this could serve as a wakeup call for a team comfortably at the top of the National League Central.
"Sometimes you need it," he said. "You don't want it. As bad as [the Astros have] been playing, they need some kind of luck, too. It just wasn't our night tonight, and it was theirs."