Basically, it means Kordell Stewart has been benched, although he is not fully recovered from a leg injury sustained two weeks ago against Oakland and aggravated in last Sunday's 20-13 loss at New Orleans.
Coach Dick Jauron did not elaborate on the switch.
"It's a question that doesn't need to be answered really," Jauron said. "The fact is that Kordell is hurt and Chris is playing this week."
Stewart was 10-of-21 for 152 yards with one touchdown against the Saints, but lost two key fumbles in the fourth quarter.
Chandler has not started a regular season game since the next-to-last contest last year. In a 24-14 loss at Carolina, he completed 7-of-8 passes for 113 yards before being knocked out with his fifth concussion in six years midway through the second quarter.
The Bears will try to snap a 10-game road losing streak at Seattle.