Adam Schefter reports on Matthew Stafford's injury that will keep him out of the Lions' matchup with the Bears. (1:05)

CHICAGO - For the first time since the start of the 2011 season, Matthew Stafford came out of a tunnel into an NFL stadium on Sunday afternoon wearing not his No. 9 jersey, but a pair of warm-ups and knit cap atop his head. The Detroit Lions quarterback had started 136 straight games, the second-longest active streak in the league, until a back injury kept him from start No. 137.

A source told ESPN's Adam Schefter that the injury involves fractured bones in his back.

So he watched. He stood on the sidelines and remained as active as possible, often attaching himself to now-backup quarterback David Blough and then conversing with starter Jeff Driskel during Detroit's 20-13 loss to Chicago. He'd get animated with offensive coordinator Darrell Bevell when they were on the sidelines together, and there were also times he moved to different position groups to give an encouraging word.

It was a different place for the 31-year-old and it was clear he was trying to adapt the best he could after dealing with a back injury that Lions coaches and doctors deemed it unsafe for him to play with. The Lions said Stafford injured his back on a hit late in the fourth quarter against Oakland -- a drive that ended when he almost threw a game-tying touchdown pass to Logan Thomas that fell incomplete -- and then was limited in practice all week.

"We were hoping by the time we got to today that we were going to be able to do it and like I said, it just wasn't safe to put him in that situation," Lions coach Matt Patricia said. "So the information that we got, talking with the doctors, it wasn't the right thing to do. Again, I'm more concerned about him more than anything else. He's got a great family. He's a great guy.

"So for us, that was a decision that we made and we moved forward and the team, you know, they moved forward with the decision. And I think a lot of those guys were trying to fight and do everything they could because they know how tough he is, and they know how much it means to him and I think they were trying to do everything we could for him."

Patricia did not delve into the specifics of Stafford's back injury and would not give a timetable for the return of Detroit's starting quarterback, but a source told Schefter that the injury could sideline Stafford for "this week or three weeks." Stafford is in the midst of one of the best seasons of his career, throwing for 2,499 yards, 19 touchdowns and five interceptions.

"It's day-by-day, week-by-week," Patricia said. "And we'll see how it goes from that aspect of it and see how everything progresses, I guess."

Most consecutive starts by QB,

NFL history Matthew had started 136 consecutive regular-season games, the sixth-longest streak for a quarterback in NFL history. QB Starts Brett Favre 297 Philip Rivers 218* Eli Manning 210 Peyton Manning 208 Matt Ryan 154 Matthew Stafford 136 * Active streak

-- ESPN Stats & Information

Patricia said Sunday was the day they decided it would be unsafe for him to play, even though he was listed as questionable on the injury report Friday. The Lions head coach wouldn't explain if anything changed for Stafford between Friday and Sunday other than they met with the doctors for one more consultation and decided "that, hey, that's not a good thing to do."

When players were told was another question. While Patricia said the decision was made Sunday, some Lions players said they knew about the quarterback change on Saturday. Offensive lineman Joe Dahl said he found out "last night, maybe," and tight end Logan Thomas said Patricia "let us know yesterday," meaning Saturday. Offensive tackle Taylor Decker deemed it a surprise and that they found out Saturday.

Lions players also said Stafford looked fine throwing the ball during the week -- but that if he were injured they likely wouldn't know it anyway. Cornerback Darius Slay said Stafford looked "awesome to me" in practice but that he wasn't surprised Stafford didn't play because "if he can't go, he can't go. That's just him as a person. I more care about his safety than him playing." Lions players know if Stafford was able to play, he'd be out there.

"Matt could be in a wheelchair and get thrown out there and stand up there and throw a pass and he'll say he's fine and ready to go," Thomas said. "He'll never show weakness at any point. That's just who he is. You've got to applaud the guy. We didn't find out what actually the problem was until [Saturday]. He's a competitor. He's a warrior. He wanted to be out there."

Thomas said Patricia addressed the team and that the message was, essentially, "go play" and that the only thing on their mind was winning. But they also recognized there would be a difference going from Stafford, who is the franchise leader in almost every major passing category, and Driskel, who was making his sixth career start and first in Detroit.

Stafford also ended his start streak, which will finish as the sixth-longest among quarterbacks in NFL history behind Brett Favre (297), Philip Rivers (218), Eli Manning (210), Peyton Manning (208) and Matt Ryan (163).

"It's pretty tough to replace a guy like Stafford," Thomas said. "He's been in the league 10 years now and played great football for 10 years. He's an elite quarterback, so obviously I think things changed a little bit, but when you lose a guy like that, what do you expect?"

Driskel held up OK in his Lions debut, completing 27 of 46 passes for 269 yards, one touchdown and one interception. Most of those yards came in the fourth quarter, including his best pass of the day, a 47-yard touchdown pass to Kenny Golladay that was one of his few, true downfield throws all afternoon.

Driskel said Stafford offered him support on the sideline throughout Sunday's game and that he's helped him learn the nuance of the game since Driskel signed with Detroit in September. Driskel said his week leading up to Sunday was a "normal week" and got a few reps here and there, but didn't make it sound like he prepared to be the starter.

Then he got the nod.

"I was excited to get out there and play," Driskel said. "And compete."

The question for the Lions now is when might Stafford get healthy, or will Driskel be Detroit's quarterback for the foreseeable future.