Rookie Pete Kozma is proving to be a more than adequate replacement.
Yadier Molina added a solo shot as the Cardinals moved 3 1/2 games in front of Milwaukee and Los Angeles for the second NL wild card. The Brewers lost 12-2 at Washington earlier in the day, and the Dodgers were off.
"I don't think anybody would look past us losing our starting shortstop, an All-Star, leadoff hitter for most of the season, and think that was not going to be impactful on our club," manager Mike Matheny said. "Pete was given the opportunity and he just jumped on it. That's how most careers are started."
Lynn (17-7) struck out nine and allowed one run and five hits. The right-hander is 4-0 with a sparkling 0.89 ERA in his last four outings, including a relief appearance against Milwaukee on Sept. 9.
Three of his hits and both walks came in the first two innings.
"He was in hot water the first two, guys all over the bases, but he worked his way through it and got better as he went," Matheny said. "It's just impressive when he gets his fastball going."
Kozma extended St. Louis' lead to 3-0 with his second major league homer in the second, a day after hitting his first one. Kozma, who is batting .298 in 18 games, said he's been helped by the encouragement of his teammates.
"There's a little extra, all the guys, you hear them cheering all the time, especially here because there's not that many fans here, so you can really hear them," he said.
Molina hit his 21st homer to make it 4-1 in the fourth.
Houston starter Fernando Abad (0-6) pitched 1 2/3 innings before leaving with a strained right oblique. He walked one and allowed a run. He was replaced by Fernando Rodriguez, who was charged with three runs and seven hits in a career-long 4 1/3 innings.
Lynn improved to 4-0 in five career appearances against the Astros, and St. Louis beat Houston for the ninth straight time in front of a crowd of 12,584 -- the smallest crowd in the history of the 12-year-old ballpark.
Daniel Descalso got St. Louis off to a fast start with a two-out RBI triple in the second inning. Kozma's homer bounced off the wall above the Crawford Boxes in left field.
Molina looked healthy a day after he was out of the lineup with lower back spasms. The All-Star catcher was injured avoiding a pitch on Saturday. He fell face down to the ground in the seventh inning getting out of the way of an inside pitch, but jumped right back up, before eventually drawing a walk.
Matheny wasn't surprised that Molina missed just one game with his injury.
"He heals quick and he lies really good too when it comes to what it really feels like," Matheny said with a smile. "He's just got a pain tolerance that's pretty impressive."
Matt Holliday helped the Cardinals put the game away with a two-run single in the seventh. Holliday, who left Sunday's game in the ninth inning with tightness in his back, went 1 for 5 with three strikeouts.
Jason Motte, who leads the NL with 40 saves, got a break after converting opportunities in each of the Cardinals' previous seven wins to earn NL player of the week honors on Monday.
Center fielder Jon Jay, who doesn't have an error this season, robbed Castro of a hit in the fourth inning with a tricky catch. He leapt to stop it with his glove, but it bounced off. He got his bare hand on it, before bobbling it and falling to his knees to snag it before it hit the ground. He smiled and shook his head as he got to his feet as players in the St. Louis dugout raised their arms and cheered.
"He continues to make catches that make you notice him," Matheny said.
Monday was the second time in the last four games that a Houston starting pitcher lasted 1 2/3 innings before leaving with an injury. Edgar Gonzalez strained his right hamstring and left at the same point in Friday's game.
Cardinals 3B David Freese was on the lineup card, but was taken out after rolling his right ankle in pregame warmups in the batting cage. He was replaced by Matt Carpenter. ... Astros 3B Matt Dominguez was scratched because of a stomach virus. He was replaced by Brandon Laird. ... St. Louis 1B Lance Berkman, who had his second knee surgery of the season on Sept. 11, said there was a "slim chance" he could play again this season. "I'm not ruling it out," he said. ... Houston OF J.D. Martinez will have surgery to repair an injured hamate bone in his left wrist on Wednesday. ... These teams continue the series on Tuesday when St. Louis left-hander Jaime Garcia opposes rookie Lucas Harrell.