"You realize that if the starter goes down, boom, you have to be in the game. You're extremely cold, you're not loose," Matthews said. "But as a backup quarterback you're over there just shooting the breeze with the third string guy most of the game.
"Your natural instincts take over. You may not be loose. You play the game for so long. Just go out there and have fun.
"I felt good basically in the second half whenever I came out and got to warm up and stretch a little bit. My first drive when we went in there we drove down and scored a touchdown but I was stiff as a board."
Matthews was named NFC Offensive Player of the Week for his performance against the 49ers. He completed 25 of 31 passes for 166 yards and a career-high three TDs after replacing Miller. In Week 2, Miller rallied the Bears to a 17-10 win over Minnesota with two fourth TD passes after Matthews exited with a strained muscle in his rib cage.
Offensive coordinator John Shoop said his game-plan doesn't change dramatically with Miller or Matthews on the field.
"Those two really have more similarities than differences," Shoop said. "They're both real accurate. When Shane went into the game last week, our game plan didn't change at all. And when Jim replaced Shane (against Minnesota), the game plan didn't change a bit. I called the game the same way.
"They're just expected to complete every pass that's called and get us in and out of good runs and bad runs. It really hasn't affected us much at all, if any.
"The offense has confidence in whichever guy ends up taking the snaps and I sure as heck have confidence in whichever guy as well."
Danny Wuerffel, who sustained a pinch nerve in his neck last week lifting weights, will be the third quarterback.