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* UPDATED 7:17 P.M. MDT

ENGLEWOOD, Colo. -- Linebacker Danny Trevathan was carted off the practice field Tuesday morning with what was later diagnosed as a left medial tibial impaction fracture.

The injury is expected to sideline Trevathan for six to eight weeks. That would project to a return for the Week 5 game against Arizona or the Week 6 trip to New York to face the Jets. The Broncos have a bye in Week 4.

"There could have been ligament damage, but there was not," said Head Coach John Fox.

Trevathan was caught up in a thicket of Broncos chasing a sweep to the right during a team period, and fell to the ground. He rolled and yelled in frustration and pain as team medical personnel rushed to treat him. He was examined for several minutes before being helped onto a cart, which took him back to the locker room.

The third-year weakside linebacker was unable to put weight on his left leg as he was helped from the cart into the main building of the team facility. Given the worst fears at the time Trevathan went down, an absence that only costs him a handful of regular-season games would be a relief to the Broncos. 

After Trevathan left practice, Marshall stepped in for him on the weak side, befitting his depth-chart spot as the No. 2 weakside linebacker.

"We were hoping it was a bruise; that would have been the best news," said Fox. "Obviously (the fracture) is better than ligament damage; that's an out-for-the-season-type thing."

Marshall joined the Broncos' practice squad last year before being promoted to the 53-man roster after Von Miller suffered a season-ending torn anterior cruciate ligament. The daily experience learning the defense last year was invaluable as Marshall stepped into more exensitve work.

"Now, I'm familiar with the defense, I have a couple of years in the league, so I know what the NFL is about," Marshall said. "So I'm definitely ready if the time was to come."

Barrow also saw some additional work with the first unit later in practice.

"I'm definitely going to try to take advantage of the opportunity," Barrow said. "I think my sense of urgency has to improve. Getting reps with the ones, I'm getting better every day. I took some reps today with Danny being down, just to get acclimated with those guys."

Although the depth chart lists Barrow as the No. 2 middle linebacker and Marshall as the second-team outside linebacker, the members of the position group are learning versatility.

"(Linebackers) Coach (Richard) Smith and Coach (Jack) Del Rio, they've been cross-training us. We've all been learning all of the positions," Barrow said.

Tuesday's injury was not the first scary moment in practice for Trevathan. Last year, he suffered a knee injury during practice four days before the Broncos' Week 5 win at Dallas. Trevathan recovered in time to play, and turned in the signature moment of his breakout season, a lunging interception of Tony Romo that set up Matt Prater's game-winning field goal.

Big plays like that helped Trevathan emerge as a defensive leader, a status that has defined his work this summer. Barrow and other linebackers are learning from Trevathan's example.

"He taught me to just be patient," Barrow said. "When I got here, I was kind of hard on myself, just trying to pick up everything as far as the defense. I wanted to accelerate faster than what I was doing.

"Danny just told me, 'Chill. Just let it come to you.' I think I've taken his advice, and every day I've come out here, I've gotten better at something."

And for the next six to eight weeks, Marshall, Barrow and other linebackers will try to apply the lessons learned from Trevathan, and play as he would.

"We'll hold down the fort until he gets back," Fox said.