Fisher, an undrafted free agent from Notre Dame, replaced Ahman Green, who hurt his left knee in the third quarter. Fisher ran 17 times for 91 yards and a touchdown. He accounted for half of the Packers' amazing 188-yard ground explosion.
Another huge rookie performance came from a more expected source. Javon Walker potentially saved the game with a touchdown-saving tackle on a wild play before halftime.
On freezing cold day at Lambeau Field, Brett Favre threw two second-half touchdown passes and improved to 33-0 at home in temperatures of 34 or below. He beat the Bears for the 18th time in 22 tries as the Packers emerged as the last team unbeaten at home.
The Packers trailed 14-6 at halftime and were on the verge of going down by another touchdown as the Bears were poised first-and-goal on the 1. The big play the Packers needed came on the next play when Rod Walker recovered Jim Miller's fumbled snap at the Green Bay 1 early in the third quarter.
The reversal of fortune came one play after Marty Booker's apparent touchdown catch was called an incompletion, and the Bears lost a challenge.
The Packers then drove 90 yards -- with Fisher accounting for half of them -- and got their first lead on Ryan Longwell's 27-yard field goal that made it 16-14. They never relinquished the reigns.
Favre added his second TD pass of the day on an 8-yard toss to Bubba Franks, and Fisher scored from 2 yards out with 1:10 left to make it 30-14.
Former Packer Henry Burris threw a 45-yard touchdown pass to Marcus Robinson with 8 seconds left.
Fisher was third on the depth chart until fellow rookie Najeh Davenport was lost for the season two weeks ago with a fractured left eye socket. Starter Ahman Green suffered a bruised knee cap on a play in which he was whipped down by the face mask after a 39-yard run. Fisher stepped in -- and stepped up.
The Bears (3-9) also lost their starting running back, Anthony Thomas, who broke his right index finger in the first half. He was replaced by Leon Johnson.
Green hurt his left knee when he was tackled by Mike Brown at the end of a 29-yard run early in the third quarter.
The Bears took a 14-6 halftime lead on tight end Dustin Lyman's first two career touchdowns, one of which he scored on a fake field goal.
Playing on grass for the first time all season, the Bears went ahead 7-0 when Paul Edinger lined up for a 30-yard field goal in the first quarter and holder Brad Maynard pitched the ball to Lyman instead.
The TD was set up by defensive end Alex Brown's recovery of Favre's fumble at the Green Bay 18. Miller's 8-yard touchdown toss to Lyman made it 14-3, and Longwell's 31-yarder made it 14-6.
The balance of the game switched several times in a crazy final minute of the first half.
Johnson fumbled at midfield with 21 seconds left, and Na'il Diggs recovered for Green Bay. On the final play of the half, Favre's pass was intercepted at the Bears 5 by Damon Moore, who raced down the sideline but fumbled at midfield.
Packers lineman Mike Flanagan recovered but tossed the ball into the air. Chicago cornerback Roosevelt Williams grabbed the gift and was on his way to another Bears touchdown when Javon Walker chased him down 15 yards shy of the end zone.