With Wagner unavailable because of the left oblique injury he suffered on Friday night in Game 2, Cox gave Kimbrel a chance to preserve the one-run ninth-inning lead the Braves held in Game 3 of the National League Division Series on Sunday night at Turner Field.
But with runners at first and second and two outs, Cox removed Kimbrel in favor of fellow rookie reliever Mike Dunn, who lost his lefty-lefty matchup against Aubrey Huff. After Huff delivered a game-tying single, Buster Posey delivered a decisive grounder through Brooks Conrad's legs and allowed the Giants to take command of the best-of-five series with a 3-2 win.
"We had this one won," Cox said. "We gave up a walk, a base hit and a base hit.
"You know, we're not the best team in baseball. But we can win games and we can compete against anybody. But we can't afford to make mistakes."
Conrad's third error of the night only magnified the fact that the Braves looked lost in their first game without Wagner. Kimbrel retired two of the first three batters he faced before allowing a one-out single to Freddy Sanchez.
After Huff singled off Dunn and Peter Moylan induced the potential inning-ending grounder off Posey's bat, Kyle Farnsworth ended the ninth-inning madness by striking out Nate Schierholtz.
"We shouldn't have been in that situation," Kimbrel said. "I should have gotten them out before then."
Kimbrel and Dunn spent most of this season with Triple-A Gwinnett. But with Wagner and valuable left-handed reliever Eric O'Flaherty injured, the two rookies were asked to complete an inning that left the Braves with absolutely no room for error.
Down 2-1 in this best-of-five NLDS, they can only hope that the following days they spend without Wagner prove to be more gratifying than this first one was.
"We have an attitude that that's what we need to do," Moylan said. "We have no other choice but to do that. You can't just say, 'Oh Wags is down, let's just quit.' We've got arms down there that can get the job done, and we'll see it these next couple of days."
When Wagner suffered his injury, Cox said that he would essentially use a closer-by-committee approach that would utilize Kimbrel, Moylan, Dunn, Farnsworth and Jonny Venters based on the situation.
Kimbrel seemed to be the best candidate, especially after he kept Game 2 tied with two scoreless innings in front of the raucous San Francisco crowd Friday night. The 22-year-old flamethrower entered Sunday having recorded 28 strikeouts in the 14 scoreless innings he had worked dating back to Sept. 9.
"Our bullpen has been good all year," said Tim Hudson after allowing just an unearned run in seven innings. "It's something that once you get the lead late, you just have to hit the easy button and take it to the locker room. That just wasn't the case today."
Mark Bowman is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.