The Astros have lost five in a row and nine of their last 10, dropping their 12th consecutive series in the process.
Rangers starter Martin Perez matched Keuchel all the way, tossing his first career complete game to overcome his counterpart's return to the Houston rotation.
"Everything," Keuchel said, when asked what was working for him. "It's the best I've felt all year. Perez was doing his thing over there and it made it easier, because the game was going so fast I didn't have time to get tight. He pitched a hell of a game."
Making his first start since July 27, Keuchel cruised through the first four innings without allowing a baserunner. With one out in the fifth, Alex Rios reached on an infield single that could have been ruled an error with shortstop Jake Elmore double pumping before the throw.
The window opened just enough for the Rangers to sneak through, as Jeff Baker followed with a single before Geovany Soto boomed a ground-rule double to plate a run.
More of the same followed in the sixth, as Elvis Andrus reached base on a throwing error from Elmore. It was the second error in as many games for Elmore, who is filling in at shortstop for the injured Jonathan Villar.
After an Adrian Beltre single, Rios doubled Andrus home to give Perez an unshakeable lead.
"You can't give outs away," said manager Bo Porter. "You look at the first two runs they had, it was in innings we gave them an extra out and they took advantage and were able to score.
"If not for that, we may be going into the ninth with the score tied 0-0, and we may be still playing, or [Chris] Carter hits a walk-off homer. At the end of the day, you have to play fundamentally sound baseball."
While the Astros were giving outs away, Perez kept getting them, breezing through an aggressive lineup as the game clocked in at just under 2 1/2 hours.
"I definitely think we were overaggressive," Porter said. "When you have a guy working at the pace he was working, it would be a good idea to slow it down and make him throw a little bit more pitches and get the pace of the game in your favor. But it felt like the more outs he recorded, the quicker our at-bats got."
The Rangers pulled away in the ninth, as Jurickson Profar slapped a single to right that plated two runs before right fielder L.J. Hoes' throw sailed into the seats behind the visiting dugout, scoring another run.
Keuchel still pushed on, though, finally exiting after allowing another run on Leonys Martin's single.
"Perez was just better than me today. I didn't make all the pitches when I needed to," Keuchel said. "Very frustrating. It felt like the [July 27] Toronto game, when I made a good pitch and [Jose] Bautista hit it out. This time it was Profar. I'd like to have the hit-by-pitch back to [Craig] Gentry before. Felt like I could have gotten him out."
Though the southpaw faltered late, Porter said there were no qualifiers needed to express his approval of Keuchel's outing.
"I was pleased with Keuchy's performance all the way through," Porter said. "Even in the last inning, to me, there were some plays that could have been made."
Perez sprinkled a few early hits, but the Astros never pushed a runner past second base on him until Carter homered with two outs in the ninth. The lefty allowed four hits and two walks while striking out eight.
For Carter, it was his second straight game with a home run after being mired in a 3-for-32 slump.
"It was a middle-away changeup, and I just kind of hit it one-handed almost," Carter said. "That's the whole approach with what I've been working on. Not getting beat by balls and staying balanced behind it."
Even with the late runs off Keuchel, Houston starters have compiled a 3.69 ERA over the last 11 games, but Brett Oberholtzer has the only wins during that span.
Though those performances haven't translated to wins, Texas manager Ron Washington has come away impressed with the Astros' young arms.
"That's what you go through with a young team," Washington said. "They're trying hard and they're trying to experience how to be Major League players. That guy yesterday, [Brad] Peacock, and Keuchel, they are going to pay dividends down the road. Offensively, they've got enough to score runs. They've just got to straighten out a few things, and they will before long."
The Astros wrap up a seven-game homestand on Monday, as they look to avoid a four-game sweep to their Silver Boot series rivals.