The X-rays were negative, but the MRI revealed a fracture in his eighth rib, said Peavy, who added that it will take from four to six weeks for the injury to heal.
Three runs and six hits in the ninth -- including four hits in a row with two outs against Isringhausen before he put away Hernandez -- had the Padres convinced they can compete on the big stage with a powerhouse that won 18 more games than they did during the regular season.
"Down 8-0 against those guys and having an opportunity to tie it or win it -- that's a pretty good feeling," Mark Sweeney said. "Yes, we lost, but there definitely were some positives."
Sweeney doubled and scored the first Padres run in the seventh against Brad Thompson after starter Chris Carpenter departed, having endured six scoreless innings.
There would be 10 hits against the Cards' bullpen in those final three innings, with Hernandez, a man who produced several memorable game-winning blows down the stretch, poised to strike before Isringhausen struck him out with his killer cutter.
"That's not easy, coming back against their closer like that with a six-run lead," Hernandez said. "We kept battling, and that shows us that we can come back any time, even in the ninth inning. He made some good pitches on me, cutters. Tip your hat.
"It's frustrating, but they know that if it's close, they've got to be careful. We can come back."
The most serious of the Padres' postgame concerns involved the condition of Peavy, who was to have an MRI taken of his right rib cage after a rough introduction to postseason play.
Peavy came out throwing strikes. The Padres' All-Star right-hander threw 10 pitches in the first inning, all in the strike zone. His fifth strike, an 0-2 fastball off the bat of Jim Edmonds, cleared the fence in left-center. It sent the Redbirds winging toward an eight-run cushion behind a memorable performance by Reggie Sanders, who crushed a grand slam in the fourth and produced an NLDS-record six RBIs.
"We've been riding our Peavy horse all year long," said Eric Young, whose pinch-hit homer in the eighth against Randy Flores and RBI grounder in the ninth contributed heavily to the late uprising. "He's entitled to have an off-game.
"I wish we could give him the ball again on Thursday. He's a tremendous competitor, and he'll want to get back out there."
The Padres have veteran right-hander Pedro Astacio lined up for Game 2 on Thursday against southpaw Mark Mulder, with Woody Williams set for Game 3 on Saturday at PETCO Park. Adam Eaton will be rested and ready for Game 4 on Sunday.
"I can't say enough about the ballclub in the ninth inning," Padres manager Bruce Bochy said. "To be down eight runs and end up getting the winning run at the plate ... I was proud."
Early on, Bochy was frustrated. The Padres were working the count effectively against Carpenter before self-destructing with double-play grounders.
Meanwhile, the Cards were aggressively coming after Peavy. In the third, they pieced together a creative three-run inning with a flare, a bloop, a squib and a stumble.
After David Eckstein's flare to center for a single, Edmonds followed with a fly ball that fell in front of Ryan Klesko in left, near the line, for a double.
After Albert Pujols was walked intentionally, Peavy stumbled on the rubber making a delivery to Larry Walker, scoring Eckstein and moving the others up on the wild pitch. After Edmonds was walked intentionally, Sanders reached for an 0-2 pitch and rolled it inside first base. When Sweeney was unable to make the play with a backhand, the ball rolled into shallow right field, scoring two runs.
"It was playing up and I tried to backhand it," Sweeney said. "It went off the end of the glove and squirted away. It just spun off the end of his bat. That's the way baseball works."
There was nothing fluky about Sanders' third, fourth, fifth and sixth RBIs. Peavy lost command in the fifth after Edmonds grounded sharply off his glove for a one-out single. Pujols dropped a single in front of Klesko on an 0-2 pitch that missed catcher Hernandez's target on the outside corner by a foot. Walker walked on five pitches, loading the bases for Sanders.
Falling behind, 3-0, Peavy grooved the last of his 70 pitches. Given the green light by manager Tony La Russa, Sanders launched one into the red sea of fans in the left-field bleachers. It was 8-0, and Peavy was replaced by Clay Hensley.
The frustration for the Padres' offense came in the form of ground balls in all the wrong places. After Sweeney's single and an error by Eckstein on Hernandez's liner with no outs in the second, Khalil Greene flied to center and Joe Randa bounced into a double play started by Eckstein at shortstop.
In the third, Dave Roberts singled with one out and took second on Klesko's single. On a full count, after fouling three balls off, Mark Loretta banged one right to third baseman Abraham Nunez, who stepped on the bag and doubled up Loretta.
"I looked at the video, and [Nunez] was moving toward the bag with the runner going," Loretta said. "When I first hit it, I figured it was down the line for a double, maybe two runs.
"I don't want to say another team gets more breaks. They capitalized. But Jake catching his spikes? I've never seen him do that. Sanders throws his bat at the ball twice -- and the second time it went down the line and they get two runs. That's baseball. Show up and play, and see what happens."
Making three inning-ending double plays in as many innings, Eckstein turned one in the fourth on Hernandez after a Sweeney walk, stepping on the bag and doubling off the catcher easily.
With Carpenter leaving after six innings with cramping in his throwing hand due to dehydration, the Padres scored in the seventh on Sweeney's double, Hernandez's single and Greene's sacrifice fly against Thompson. Lefty Flores struck out pinch-hitter Xavier Nady to quell the threat.
Young's homer produced a run in the eighth, with the Padres leaving two runners stranded when Hernandez grounded out.
Greene's double started the ninth against Cal Eldred, who was saved on a spectacular catch in deep right by Walker on Randa's drive. That catch proved critical. Damian Jackson singled and stole second, bringing in Isringhausen. After Young's grounder scored Greene, consecutive singles by Klesko, Loretta, Giles and Robert Fick made it 8-5 with the bases full for Hernandez, who went down swinging.
"Coming back and scoring those runs late, maybe that'll make everybody more comfortable," Klesko said. "We need to take that same approach into the next game."