The game continued what has become a frustrating trend for the D-backs, who have gone 14-23 since a July 4 loss to the Dodgers.
Wednesday's loss, combined with the Rockies' win over the Braves, dropped the D-backs to a game behind Colorado for the top National League Wild Card spot. Arizona leads Milwaukee by four and St. Louis by five.
Manager Torey Lovullo, though, said it isn't time to worry.
"We have so long to go and we have a lot of the story that we've yet to tell," Lovullo said. "And I know that we aren't playing our best baseball, and I know that seasons have their ups and downs and we're not winning games right now, it's very obvious, but I think our guys are going to turn this thing around."
It looked as though they had turned Wednesday's game around when they rallied from a four-run deficit to make it 5-4, thanks to a three-run eighth that included a two-run homer from Paul Goldschmidt and a solo shot by J.D. Martinez.
Unfortunately for the D-backs, three relievers combined to give up four runs in the eighth.
"We showed a lot of battle," Lovullo said. "I know that things didn't get off the right way for us, but I think something clicked offensively for us. I think Goldy's home run set a really good tone for what we were ready to do. Unfortunately, we didn't have a shutdown inning."
Walker (6-7) did take some positives out of his outing, believing that a number of the Houston hits were not hit very hard.
The numbers from Statcast™ showed that as well, with only two hits -- Carlos Beltran's homer in the second and Alex Bregman's single later that inning -- registering an exit velocity of 100 mph or more.
"I felt like I made good pitches," Walker said. "It was just one of those games where every ground ball hit found a hole. I didn't have my best fastball velo, but I felt like I was locating pitches, and I thought my offspeed pitches were pretty good."