Theriot, who made 87 starts at shortstop before the Cardinals acquired Furcal near the trade deadline, is in the lineup at second base.
"I feel not that good about what happened yesterday," Furcal said. "It was a big disappointment, the way I played. I'm trying a little too hard."
Matt Holliday missed his ninth straight start with a tendon injury on the middle finger of his right hand, but played catch and took batting practice and was available to pinch hit. Holliday said he would start Saturday.
"It felt all right," Holliday said. "Just looking (to see) if I can take my normal swing, if I get out there and don't have to think about it."
Furcal mishandled a potential double play ball with none out and a man on in the ninth, opening the door for the New York Mets' six-run rally in an 8-6 final that dropped the Cardinals two games back of Atlanta for the NL wild card with six games to play.
Furcal has committed five errors in the past six games, all but one of them fielding miscues.
"He's a proud guy, he's trying to win," La Russa said. "That play in the ninth got to him and he's made a couple errors lately, so it's just a good day for him to get a break."
La Russa had less to say about the bullpen. Jason Motte and Fernando Salas have both been heavily used during the Cardinals' September drive. Both pitched Thursday, and the manager wouldn't say whether Motte was available.
"You know, if we get the double-play ball and a pop-up, then where is our tired bullpen?" La Russa said. "It's such a ridiculous story line and it's aggravating. It's not accurate and it's guys picking and choosing things and it's just a pain to deal with, and I'm not going to deal with it.
"Just watch the game and see who pitches."
Furcal was a .158 career hitter (6 for 38) against Cubs starter Ryan Dempster, but La Russa said that had nothing to do with his decision.
"I really think that even if he had good numbers, he was beating himself up after the game so much that he could use a break, and he'll be back in there tomorrow," the manager added.
La Russa said the team would have no trouble shaking off a loss that hurt their wild-card drive. The Cardinals had won seven of eight before missing a shot at a sweep of the Mets.
The manager didn't see players giving outfielder Shane Robinson a hot foot during the game, and didn't mind unless it happened during play.
"I guarantee you it was done by the Gashouse Gang when they won, it was done by the '64 Cardinals," La Russa said. "You know what I look at, are they playing as hard as they can, do they care about the outcome?
"That's why I don't worry too much about hair on the faces and length of hair and tattoos or nipple rings or anything."