Bears running back David Montgomery remains in the NFL's concussion protocol and has been ruled out of Monday night's game against the Vikings.

The 2019 third-round pick from Iowa State suffered a concussion late in last Sunday's loss to the Titans. Montgomery entered Week 10 ranked 13th in the NFL in rushing with a team-high 472 yards and one touchdown on 131 carries.

With Montgomery missing his first game in two seasons, the Bears will be without their top two running backs entering the year; Tarik Cohen is out for the season after tearing his ACL in a Week 3 win in Atlanta.

Other running backs on the Bears roster include converted-receiver Cordarrelle Patterson, third-year pro Ryan Nall and undrafted rookie Artavis Pierce. Nall replaced Montgomery against the Titans, catching four passes for 35 yards, including his first NFL touchdown on a 6-yard reception from Nick Foles.

"With [Montgomery] being out and being in the protocol right now," said coach Matt Nagy, "what we've seen all week in practice is just for our guys that are all there in practice to be able to kind of fill different roles.

"I think right away you look at a guy like a Ryan Nall or Cordarrelle and these guys that have been on the roster, and I think that you see that, 'Hey, it's the next-man-up mentality.' And of course the other running backs that we have here, too, they've done a good job between helping us out with the first group and helping us out, too, with the numbers that we need with the look team. It's just all of us just kind of working together and saying, 'I don't know how it's going to go, but we're going to need you guys.'"

Nagy also revealed Saturday that the Bears could activate veteran running back Lamar Miller from the practice squad. The eighth-year pro was signed to the practice squad Oct. 5 after missing the entire 2019 season with a torn ACL he sustained in a preseason game while playing for the Texans. 

"He could be an option," Nagy said. "It's something we're definitely looking into. He's done a good job of being able to continue to practice hard each and every day. We're watching him as we go in practice and same thing with a guy like Pierce, too. These guys are working hard in practice and they're giving us everything they have, and now what we've got to do is figure out roster-wise with all the moving parts, what do we want to do here in regards to that position."

Miller has appeared in 105 NFL games with 89 starts over seven seasons with the Dolphins (2012-15) and Texans (2016-18), rushing for 5,864 yards and 32 touchdowns on 1,354 carries and catching 209 passes for 1,565 yards and 8 TDs. He was voted to the Pro Bowl with Houston in 2018 after running for 973 yards.

"He's a really kid good that has a lot of experience and he has had that [torn ACL] that is not always easy to come back from," Nagy said. "But when he was playing at his highest level, he was one of the best running backs in the NFL. And to have a guy like that, to me, there's an excitement of the possibility of him being able to fill a role for us. I do love this story. I think that if he does get a chance to go out there and do something, it will be exciting for him and for us."

In other injury news, defensive tackle John Jenkins (ankle), special-teams standout Sherrick McManis (hand) and quarterback Mitchell Trubisky (right shoulder) have also been ruled out of Monday night's game.

Center Sam Mustipher (knee) is doubtful after missing practice all week, while receiver Allen Robinson II (knee), tight end Cole Kmet (groin), tackle Jason Spriggs (knee) and linebacker Barkevious Mingo (shoulder) are questionable after being limited in Saturday's workout. 

The Bears' depleted offensive line could receive a major boost, with Nagy revealing there's a chance that center Cody Whitehair could be activated off the reserve/COVID-19 list in time to play Monday night. 

"I would say he's a possibility," Nagy said. "I just don't have any answers right now, but I would say it's a possibility."

Whitehair suffered a calf injury Oct. 26 late in a loss to the Rams and then missed a game the following week against the Saints for the first time in his five-year NFL career. Mustipher, a first-year pro, made his first NFL start versus New Orleans but injured his knee.

That forced the Bears to start their third center in as many weeks in first-year pro Alex Bars last Sunday against the Titans. A natural guard, Bars performed well given that he had never previously played center. 

"Through everything that kind of went on with us struggling last week and losing that game, I got to say I was probably remiss in not mentioning how well I thought that Alex played in that situation," Nagy said. "Really, really proud of the way he handled that. That is not easy. He did a great job. Kid is such a true professional. Takes the game very seriously. I thought he did a really, really good job, as did the rest of the coaches. Again, depending on what happens with Cody's situation, with Alex, we're lucky to have him."