"We need around 100 [pitches] from him, no matter what," Gibson said before the contest.
That "no matter what" turned out to be the problem.
Kennedy gave up a career-high 10 earned runs on a career-worst 13 hits as the Cardinals used an eight-run fourth inning to roll to an easy 12-8 win before 43,798 at Busch Stadium.
The D-backs (34-26) had won four of their previous five games. They closed an eight-game road trip with a 4-4 mark and remain on top of the NL West.
But the nine-day swing ended on a sour note, with Kennedy taking one for the team in a rough four-inning, 99-pitch effort.
Gibson was hoping Kennedy (3-4) could eat up some innings and help out a beleaguered bullpen that needed 12 relief appearances over the previous three games of the four-game set.
Instead, Kennedy gave up a career-worst eight runs in a disastrous fourth inning. Matt Adams smashed a three-run homer and Daniel Descalso added a two-run shot as 11 hitters came to the plate and the Cardinals bolted to a 12-2 lead. The usually efficient Kennedy needed 50 pitches to get out of the frame. He also threw wide of second base on a potential double-play ball that could have ended the inning.
Gibson hated to leave Kennedy out there during the 25-minute half inning, when it was obvious that Kennedy was struggling.
"I didn't have a choice," Gibson said. "It happened quick. You wouldn't expect it to get away from him -- but it did. You don't ever want to leave anybody in that long."
Kennedy felt as though he let down his bullpen buddies.
"It stinks. Both bullpens have been taxed and I really wanted to go out there and go deep in the ballgame," the righty said. "They save a lot of games for us. They do a good job. I don't like doing that to them."
The key to the inning may have been the botched double-play ball. Kennedy cleanly fielded the grounder off the bat of Yadier Molina, but got confused as to who was covering the bag at second. Kennedy threw the ball wide of the base, and shortstop Didi Gregorius took the toss but was unable to touch the bag before throwing on to first to get Molina.
St. Louis went on to score seven runs after the mixup.
"We've made that play and we expect to make that play," Gibson said. "We just didn't execute."
Kennedy took the blame for the errant toss.
"It was my fault," he said. "Things might have been different if we turned it."
The eight runs were the most the D-backs have given up in an inning this season.
"You throw up eight in an inning against any big league team, that's pretty special," St. Louis manager Mike Matheny said. "And once again, against a good pitcher."
St. Louis rookie right-hander Shelby Miller (7-3) silenced the red-hot Arizona hitters, allowing two runs on six hits in a six-inning effort. He struck out nine.
The D-backs had scored 38 runs in their previous five games.
First baseman Paul Goldschmidt provided an early offensive highlight with a groundout that brought in Gerardo Parra with the game's first run. The red-hot Goldschmidt has driven in a run in each of his last six games, a career best.
Parra got the night started on the right foot for Arizona with a double off the wall in center. Gregorius followed with a hit to left to set up Goldschmidt for his NL-leading 54th RBI. Miguel Montero followed with a single to right to push the lead to 2-0.
But St. Louis tied the game in the bottom of the inning on a two-run homer by Matt Holliday.
Kennedy gave up two hits in both the second and third, yet managed to wiggle out of trouble before the horrific fourth frame.
Reliever Matt Reynolds surrendered back-to-back homers to Miller and Matt Carpenter in the fifth, as the Cardinals pushed their lead to 12-2.
Arizona bunched together four successive hits to score three times in the seventh. Wil Nieves, Cliff Pennington and Parra drove in runs. The D-backs scored three times in the ninth on three walks and a double by Nieves. But St. Louis closer Edward Mujica got Josh Wilson to strike out to end the game.
"We kept fighting to the end -- that's just the type of team we are," Goldschmidt said. "We'll bounce back."
Gibson said he was pleased with the road trip, which featured a rainout in Texas and a 14-inning game in St. Louis.
"It's fine, the guys played hard," the manager said. "It was kind of an interesting, long trip."
Arizona finishes the season with a 4-3 mark against the Cardinals. Gibson, for one, would love to see a postseason rematch between the teams.
"That would be great. I enjoy playing against them," he said. "They do it right. Hope to see them again."
Parra, in right field, turned in the defensive gem of the game with a full-out diving catch of a drive by Carpenter in the first.
Arizona begins a brief three-game homestand on Friday against San Francisco before embarking on a six-game road trip.