McCown, who signed a one-year contract last month, was surprised upon learning he would start an NFL game for the first time since 2007.
"From where I was sitting," he said Wednesday, "it didn't look like it was going down this road."
He served as the backup to Hanie, who completed just 50 percent of his passes in four straight Bears losses.
Suddenly thrust into a starting role, McCown said it wasn't "ideal" to take over this late in the season.
"But at the same time, as a competitor ... you have to get yourself up to speed and get ready to play," said McCown. "Everybody expects you to go play and play well."
McCown has played for four NFL teams since being taken in the third round of the 2002 draft by the Arizona Cardinals. He played for Bears offensive coordinator Mike Martz with the Lions in 2006.