Manager Bruce Bochy also held out hope that Peavy could start again, possibly in Game 5 in the National League Division Series -- if it goes that far.
Pedro Astacio pitches for the Padres in Game 2 on Thursday, trying to get them even after their 8-5 Game 1 loss on Tuesday. Southpaw Mark Mulder will face a lineup with eight right-handed bats, with center fielder Brian Giles serving as the only lefty.
Veteran right-handers Woody Williams and Adam Eaton are scheduled to work Games 3 and 4, if necessary, at PETCO Park on Saturday and Sunday.
"Any time you have a cracked rib, the guy's going to be doubtful," Bochy said. "At the same time, I think you can stay optimistic that after a couple days, when this subsides, that maybe it's not quite as bad as we may think.
"We rely on our medical people. We have our doctors here, and I think between them and how Jake is feeling, we can decide which way to go."
It still is unclear when he actually fractured the eighth rib on his right side. There apparently was some initial damage done to the rib cage during the division-clinching celebration in the PETCO Park infield last Wednesday, but the medical staff told Peavy he could not have prepared the way he did in the following days for the NLDS start if the rib had been fractured.
He believes much of the damage was done when he tried to complete his follow through after catching his spikes on the rubber making a wild pitch in the third inning on Tuesday, as the Cards were scoring three runs to take a 4-0 lead.
"They feel strongly that I broke the rib during the game," Peavy said. "In the third inning, that [wild pitch] caused more pain than I was in. I knew I hurt it on that pitch."
Just as he embraced the burden of carrying the Padres' hopes as their No. 1 starter, Peavy absorbed responsibility for the Game 1 loss.
"If there's anybody to blame for this," Peavy said, "It's me. I underestimated the extent of it."
As they prepared to engage the Cardinals in Game 2, the Padres were pointing no fingers in Peavy's direction. On the contrary, they were applauding his effort and willingness to go after the heavy-handed Redbirds in a somewhat diminished condition.
"Jake felt he could compete and make pitches," second baseman Mark Loretta said. "If he gets a few breaks -- he had some bad luck in that third inning -- and cruises through, it's a whole different story. Everyone's talking about how competitive he was.
"I'm out there right behind him, and I couldn't determine that anything was bothering him. If you have an injury that's debilitating, there are going to be signs. He's going to be wincing. He's not going to have any command at all.
"I thought Jake was throwing well. His command was excellent. I thought he had good stuff, too. He was getting up in the 90s. He didn't look like a guy who was hurting to me.
"The point is, you can't necessarily let the results be the judgment. Things happened that were out of his control as a pitcher."
Peavy surrendered a first-inning homer to Jim Edmonds on an 0-2 fastball. All 10 pitches he threw in that inning were strikes.
The hardest hit ball in a three-run third inning was a double-play grounder by Mark Grudzielanek that ended it.
David Eckstein flared a single to center. Edmonds lofted a pop fly that fell near the left-field foul line for a double. The big moment in the inning came when Peavy caught his spikes on the rubber, stumbled and uncorked a wild pitch to Larry Walker. It is believed his unorthodox movement in making the pitch put uncommon strain on the rib cage.
In the fifth, singles by Edmonds and Albert Pujols and a walk to Walker preceded Reggie Sanders' grand slam on a 3-0 fastball that Peavy left in the middle of the hitting zone, his last of 70 pitches.
"If I knew it was going to be as serious as it is and affect my pitch quality," Peavy said, "I would have said something and given the ball to somebody more capable.
"It's tough to get hit around the ballpark and say you're hurt. It doesn't look good. I didn't know the extent of the injury, that it was as bad as it is.
"I thought if I'd get some outs, and we'd have won the game, nobody would be talking about it now."
The Padres have watched Peavy pitch gamely through numerous ailments and injuries this season.
"That's Jake," shortstop Khalil Greene said. "That's his nature. He's a competitive guy, and he wants to be out there.
"I thought he had a pretty good game, actually. I didn't see anything wrong with him from where I was. From a performance standpoint, he looked pretty comparable to what I've seen in the past.
"It's not like they scalded a lot of balls, aside from the two home runs. And that can happen any time with good hitters."
Peavy was weakened for a month by an upper respiratory condition but missed only one start. He pitched in Washington two days after damaging his right middle finger, discoloring the nail and causing swelling, in a batting practice mishap -- and delivered a five-hit shutout.
"Everybody's been out there hurt," Peavy said. "I wasn't going to walk off the field in a game of that magnitude."