ENGLEWOOD, Colo. -- Every year, almost without fail, an NFL team can count on one or two position groups being beset with injuries far above the norm.

On offense, that position is running back, where the top two players on the depth chart, Montee Ball and Ronnie Hillman, have missed a combined eight games. On defense, it's linebacker, where Brandon Marshall became the latest to succumb when he suffered a concussion late in last weekend's 39-36 win over Miami.

With Nate Irving and Danny Trevathan on injured reserve, the Broncos are stretched thin. Trevathan returned to practice Wednesday, but he is ineligible to come off injured reserve until Week 15 against San Diego. During Wednesday's practice, the crop of weakside and middle linebackers who took part and can play in Kansas City included three rookies and third-year veteran Steven Johnson, who replaced Irving as the middle linebacker in the base package.

Even with safety T.J. Ward factored in because of his recent work as one of two linebackers in the nickel package, the Broncos' depth is relatively untested. Further, it might take multiple players to replace Marshall, given his every-down role as a weakside linebacker in the base defense and one of two nickel linebackers, duties he's handled with ever-increasing efficiency.

"Yeah, I think we've got a long list," Head Coach John Fox said. "There's some young guys obviously. We've been without Danny Trevathan for a long time. You know that's what gave Brandon Marshall his opportunity so we've leaned on Corey Nelson, Lamin Barrow, Steven Johnson."

Barrow and Nelson have experience working in the nickel package next to Marshall in the last several weeks, with Nelson working extensively there against the Jets in Week 6 and Barrow receiving first-half work against the Dolphins. Barrow also worked as one of two nickel linebackers on the final series last Sunday after Marshall's injury.

But the wild card might be Todd Davis, an undrafted rookie of whom the Broncos thought so much that they pounced to claim him off waivers on Nov. 13 when the Saints tried to pass him through to their practice squad.

"We've got Todd Davis out there [Wednesday], a guy we acquired last week that showed some quickness in learning in the [special-teams] stuff and he got a pretty good look [during Wednesday's practice," Fox said.

Davis has been busy the last two weeks learning the scheme -- and multiple roles in it. The former Sacramento State standout said he's been playing "all over" the linebacking corps to help him get a grasp on multiple sets of responsibilities.

"I definitely feel like I have a pretty good grasp of the defense, and it's only going to get better," Davis said. "It's an everyday process. It's a learning process, and by Sunday, I feel like I'll be right where I need to be, and I'm very excited."

Meanwhile, Johnson hopes his role can expand beyond being a middle linebacker in the base package, which he took over after Irving's injury against the Patriots on Nov. 2.

"I can handle it," said Johnson, whose goal-line stuff of Kansas City running back Jamaal Charles last year remains one of his signature plays. "I always think back to college. In college, I was that guy, I never wanted to come off the field, and it's like that right now: I never really want to come off the field. I'm wanting to play all defense and all special teams. That's just the type of player I am."

Although Johnson aches for more work, he also knows that he is an elder statesman of sorts among the linebacking corps. When any of the rookies ask for advice, he happily delivers it.

"Yeah, because I've kind of been known as a guy that knows everything, knows every position, so I tell them, 'If you've got a question, just come to me,'" Johnson said. "By no means am I perfect. Sometimes I'll go out there and I'll be like, 'Wait,' and scratch my head, but at that time, they might help me out. It's just back-and-forth communication."

And through that, he likes what he sees from Barrow, Nelson and Davis.

"They're coming along well," Johnson said. "Each and every day, you've just got to continue to get better, and everybody has the ability to be able to play in this league. Everything is just in between your ears. You've just got to be able to retain the information, understand it and put it together. Some guys learn it quick; some guys don't learn it quick and have got to do extra studying.

"We're all coming along. We all have good ability to be in this league, and we've just got to continue to work and get better."

The pertinent question for this weekend is whether "better" will be good enough to contain Charles and Knile Davis if Marshall cannot play.

"We've all got the ability," Johnson said. "Some of us have been nicked up but we're fighting through it and battling through it and at the end of it all, it'll be good."