McCown was re-signed by the Bears in March of this year to provide cover for first-string signal caller Jay Cutler.
The 34 year-old left the NFL after eight years and was coaching football at Marvin Ridge High School in North Carolina in November 2011, when he was first called by the NFC North team.
He left once Cutler returned from injury, but will now get another chance following the latter's latest setback.
McCown told Bears.com: "I'm very thankful to be here and to have my opportunity and think about how lucky I am every day when I wake up.
"I'm excited about this opportunity. In my situation, I hate the circumstances, but it's my chance to serve my team. I'm very thankful to have this chance.
"It's certainly been nice to go through the offseason this time around and be in the system, no doubt. It's a fun system for quarterbacks, it really is."
The Bears take on the Green Bay Packers next Monday night.