However on Wednesday the coach made his final call, with the newly-signed Flynn getting the nod over Tolzien, who had started last weekend in the tie with the Minnesota Vikings.
McCarthy told the club's official site: "Yes Matt Flynn will be the starter. He's taken all the reps this week and he's ready to go.
"I was focused on getting both guys ready but it will be Matt who lines up against the Detroit Lions on Thanksgiving."
The Packers are third in the NFC North, and still have hopes of making the playoffs.