The Broncos said their weakside linebacker has what's called a medial tibial impaction fracture but, fortunately, no torn ligaments.
''Well, we were hoping it was a bruise. That would have been the best news,'' coach John Fox said after the team's evening walk-through. ''But obviously, that's better than ligament damage. Those are out-for-the-season types of things. So, at some point we will get him back.''
Trevathan, who didn't put any weight on his left leg after being carted off the field during team drills, is expected to return to the Broncos right around their bye week in late September, meaning he'd only miss three regular season games, at most.
He was hurt on an 11-on-11 run play in which center Will Montgomery rolled over him.
Trevathan, a sixth-round draft choice out of Kentucky in 2012, is coming off a breakout season in which he led the Broncos with 124 tackles and intercepted three passes before leading the team with two dozen tackles in the playoffs.
''You just hate it,'' Peyton Manning said while Trevathan was being examined by the team's medical staff. ''You hate it in a game when you see that. And you hate it out here in practice. So, our prayers are with Danny. We're praying for positive results and hopefully he's OK. He's an important part of our football team.''
The Broncos were hoping for a repeat of what happened last October, when Trevathan was carted off with a scary knee injury only to return to practice that week after his MRI came back negative.
''It's pretty scary. He got carted off last year, so I'm like, 'OK, he'll be good,''' said Trevathan's backup, Brandon Marshall. ''But you never know how these things go. The defense was kind of like, 'Man, that's our guy, that's one of the playmakers on our team.' So, we all got a little worried, but we're just hoping he'll be OK.''
Before Trevathan was diagnosed with a fracture, Fox said, ''Obviously, we hope for the best. And if not, it'll be next man up.''
That's Marshall, an undrafted third-year pro from Nevada who spent most of last season on Denver's practice squad before being promoted to the active roster and playing in the regular season finale and all three playoff games.
''I guess it was like a redshirt year, so to speak. I know they don't have those in the NFL, but I feel like it was for me,'' Marshall said. ''And I feel good. On special teams I feel good, on defense I feel good.''
Marshall has already been playing alongside Trevathan in the nickel defense, a role that rookie Lamin Barrow, a fifth-round draft choice from LSU, will assume.
Asked if he felt good about the linebacker depth, which also includes rookie Corey Nelson, Fox said, ''Well, time will tell. I think in this league, it's all about opportunity, so we'll see what they do.''
What makes this injury difficult is that Trevathan is an every-down player.
''He's obviously a vital part of our defense, but not too many people heard of Danny a couple of years ago, and you never know what might happen.
Marshall said he's certain the coaching staff would allow him to make the defensive calls just as Trevathan for any time that he's filling in.
''I know I can do it,'' Marshall said. ''We only have smart linebackers on this team.''
About an hour before he got hurt, Trevathan was involved in one of three skirmishes in a four-play sequence during a four-minute drill. Finally, Fox called his players together and told them to quit losing their cool.
The Broncos added several defensive players through free agency this offseason to give them more of an edge, but the offense wasn't backing down any.
''If we're going to get hit, we might as well deliver the blow,'' said rookie running back Brennan Clay, who was at the center of a scuffle that spilled into the defensive sideline.
Manning said he's no fan of fisticuffs at training camp because fights in games only hurt your team.
''The thing about this game is it's controlled violence. At times, that's hard to control your violence,'' wide receiver Emmanuel Sanders said. ''It's good to see a team that's physical. Physicality is everything. That's what the Seattle Seahawks had last year when they won the Super Bowl.
''But it's bad because at the end of the day we're a team. So, you don't want guys hitting each other and trying to knock guys out or hurt guys out here.''
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