"Definitely disappointing," Walker said. "In that seventh inning, I just let it get away from me and left some balls up. This one's definitely on me. I should've done my job."
After Walker threw 99 pitches through six innings, manager Torey Lovullo stuck with him for the seventh and didn't have anyone warming in the bullpen. But Pablo Sandoval and Jarrett Parker hit back-to-back doubles to open the inning, leading to a mound visit from pitching coach Mike Butcher.
"We were going to go four more hitters with Taijuan," Lovullo said. "We were a little thin in the bullpen and I was trying to extend him as far as I could, and I thought he had at least three more hitters. Unfortunately, he didn't."
With Hunter Pence at the plate, Walker threw a 1-1 curveball low and in that Pence reached down and drilled to left for a two-run homer to make it a 4-3 game. The homer ended Walker's night on a bitter note.
"He's one of those guys that's really good at hitting mistakes," Walker said. "My curveball's been good all game, but just the one I left up, I got hurt by it."
Before the seventh, the 24-year-old had only allowed three hits while striking out six. The lone threat came in the third, when Joe Panik singled and Nick Hundley doubled to open the inning. But Walker got a groundout, a flyout and a popout to end the threat and proceeded to retire all but one hitter in the next three innings.
"He wiggled out of some issues in the third inning and really seemed to go into a good rhythm," Lovullo said.
Walker added, "I used my curveball more. I got some good swings and misses and got some good outs with it. The fastball command was pretty good today, and I thought we did a good job mixing in the first six innings."
With the loss, the D-Backs fell into the second National League Wild Card spot, a half-game behind the Rockies, who won Saturday. It also extended Walker's winless stretch to seven starts, as he hasn't won since June 21.
But despite the poor seventh, Lovullo believes Walker will be able to separate the positives in the first six innings from the mistakes he made in the seventh.
"In my conversations with Taijuan, he's got a very positive outlook on things," Lovullo said. "I don't think those moments are going to be a hindrance. In fact, I think they're going to help him learn and get better the next time."
Alex Simon is a reporter for MLB.com based in the Bay Area. Follow him on Twitter at @alexsimon99. This story was not subject to the approval of Major League Baseball or its clubs.