The Broncos will be without Danny Trevathan for at least the first three games of the regular season after the starting weakside linebacker suffered a fractured bone below his left knee during practice Tuesday.

Trevathan suffered what the team has called a left medial tibial impaction fracture. It’s the small bone below the kneecap. He is expected to miss 6-8 weeks, which places his return after the team’s bye week in Week 4.

The good news is Trevathan, the Broncos’ leading tackler last season, did not suffer any damage to his knee ligaments, which would have meant a longer recovery.

He suffered the injury while trying to defend a running play during an 11-on-11 drill.

Trevathan was on the ground in obvious pain for several minutes.

“I was watching the play, I didn’t see everything that happened,” said Brandon Marshall, who is the next man up to replace Trevathan. “It didn’t look good, honestly. But I’m hoping and praying it’s OK.”

After trainer Steve “Greek” Antonopulos and his assistants tried to help Trevathan up, the linebacker went back on the ground, and the cart was wheeled on the field to take him off.

After getting off the cart, Trevathan was unable to put weight on his left leg as he was helped into the trainer’s room. Broncos defensive players Derek Wolfe and Nate Irving came over to deliver some quick words of support to Trevathan, a three-down weakside linebacker who calls the plays in the defensive huddle.

The following message was sent from Trevathan’s Twitter account to his followers Tuesday afternoon: “The love is real, thank y’all! Just know that’s the only way I’m leaving that damn field!! #Godgotme #beenthroughitall #DTrevHere #Broncos”

Trevathan was also carted off the field with a knee injury during a practice last Oct. 2. That incident had a more successful conclusion. The magnetic resonance imaging exam was negative, and Trevathan played four days later at Dallas, where he made five tackles and a game-deciding interception in the Broncos’ 51-48 victory.

An overachieving sixth-round pick in the 2012 draft out of Kentucky, Trevathan went on to lead the Broncos with 84 tackles last season.

“Danny’s a fighter,” said rookie linebacker Lamin Barrow. “If it’s not broken he’s going to be back out there. It was kind of scary when he went down. You don’t really know what happened. We just got on our knees and said our prayers that he’ll be back out here.”

Marshall will be Trevathan’s immediate replacement at weakside linebacker. Selected by Jacksonville in the fifth round of the 2012 draft, Marshall spent most of last season on the Broncos’ practice squad.

Trevathan’s injury occurred after Broncos coach John Fox called his team in to remind his players there’s a fine line between playing hard and physical and putting players at risk of injury. There were several fights and skirmishes earlier in the practice.

It was after Fox called his team in and tempers calmed that Trevathan suffered his injury.

Mike Klis: mklis@denverpost.com/mikeklis