As such, the Raiders (3-3) will turn to either rookie Aidan O'Connell, who started Las Vegas' Week 4 loss at the Los Angeles Chargers with Garoppolo out with a concussion, or 15th-year veteran Brian Hoyer, who replaced the injured Garoppolo at halftime of Sunday's defeat of the New England Patriots.
Neither O'Connell nor Hoyer would say Thursday who would start for Las Vegas at the Bears.
"Yeah, just try to do my job every day," O'Connell said. "Try to get better. Definitely learn. It's a day-at-a-time process. Take it day by day, so I'm trying to do that.
"It's up to the coaches. It's how they see [the] best fit, and so, you know, I'm young, I'm definitely just taking what they say and trying to do my best, whatever that is."
O'Connell, a fourth-round pick out of Purdue, passed for 238 yards in completing 24 of 39 attempts with an interception at the Chargers. He also was sacked seven times and lost two of three fumbles.
Asked whether the experience at the Chargers should help him if he starts at the Bears, O'Connell said, "You'd hope, but it's also, it's a different scheme, different team. So again, just trying to take it one day at a time."
Hoyer said he was preparing as he does every week.
"Part of being in that role ... is you never know, and you always prepare for the best and prepare to help the team any way you can," he said.
Hoyer passed for 102 yards in completing 6 of 10 throws against the Patriots, including a 48-yard completion to rookie Tre Tucker that was the Raiders' longest play of the season.
He was asked whether he felt he had played well enough to warrant the start in Chicago.
"Well, I mean, that's not up to me," he said. "Whatever I play, I want to play good for myself and my teammates, for the coaches. So I've played for 15 years, and whatever the situation is, Sunday it is, and I'll be ready to go."
Hoyer, 38, has lost 12 straight starts, and 105 quarterbacks have won an NFL start since he was last credited with a victory, Oct. 2, 2016.
That win came at Soldier Field when Hoyer played for the Bears, one of his eight NFL teams.
"I mean, nothing ever gets any easier," Hoyer said. "I think every week is its own situation and you try to handle it the best you can, and whatever happens Sunday, you've just got to be ready. And whether it's the first snap, second half, the fourth quarter, you never know what it's going to be. So it's the role that I'm here for and hopefully do the best I can.
"My daughter does equestrian, and she gets nervous before each ride. I'm like, 'Honey, I've been playing football for 15 years and I get nervous even if I don't know if I'm going to play or not.' So I think it's not so much nervous, it's anxious. Those are good feelings. That means you care. That means you're still in it."
Garoppolo, who was injured on the Raiders' final first-half drive against the Patriots after he twisted down awkwardly and grabbed at his lower right back, did not practice Wednesday or Thursday.
He left Sunday's game at halftime by ambulance to get scanned for an internal injury. Raiders coach Josh McDaniels on Monday said it "seems like we've dodged a big bullet."
In five games, Garoppolo, who signed a three-year, $72.75 million free agent contract with the Raiders, has passed for 1,079 yards while completing 68.0% of his passes, with seven touchdowns and an NFL-high eight interceptions. He has a passer rating of 82.6 and a Total QBR of 48.3.