Favre had only limited practice this week for the Vikings while trying to rest his ailing right elbow, which has been bothered by tendinitis.
"It will be right up 'til then," Childress said. "You never know coming back. Of course, he took a few more turns (Friday). I can't look into a crystal ball and see how he wakes up tomorrow.
"He could wake up with the gout tomorrow morning," the coach jokingly added.
Favre said this week he'd consider sitting out to rest if necessary, but worse injuries haven't kept him from playing in the past. He wore a wrap on his throwing elbow, and Childress said the quarterback looked "OK" in practice.
"You're just worried about command and control of it," Childress said. "Velocity won't be an issue unless he's changing his motion."
Favre has off-the-field problems, too. He is being investigated by the NFL for allegedly sending racy e-mails and lewd photos to a Jets game hostess in 2008 when he played for New York.
Cook, whose status has become more critical with starting cornerback Cedric Griffin out for the season, said he felt "pretty good" after practice and judged his strength at about 80 percent. Cook had surgery to fix a torn meniscus in his left knee and missed Monday's loss to the Jets.
Cook also missed the first two games of the season following surgery on a torn meniscus in his right knee.
Asked if he believed he would play Sunday, Cook said, "It's a chance, but I'm not the decision maker."
Childress confirmed that Sidney Rice caught balls on the side Friday and said the wide receiver is making progress from offseason hip surgery.
"He's on two legs now, which is progress for this week," Childress said.
Childress said it still will be a couple weeks before Rice is running, and the coach expects him back "around midseason, give or take a couple weeks."
The Associated Press contributed to this report.