Trevathan, who had an MRI after he was carted off the field during Tuesday morning's practice, suffered a medial tibial impaction fracture. He will not undergo surgery.
"They told me that my ACL and MCL are fine, but that I have a fracture in my knee," Trevathan told ESPN's Josina Anderson. "The doctor told me I was going to be out four to eight weeks, but I told him four to six. I'm not tripping. I will bounce back. I'm a soldier."
Trevathan fell to the ground in pursuit of a run play during a full-team drill in the Broncos' fully padded practice. It was unclear where Trevathan made contact with another player, but backup center Will Montgomery rolled over the top of Trevathan when the two hit the ground.
Trevathan immediately grabbed for his knee and was down for several minutes before he was taken to the locker room.
"He's got an impact fracture, which is basically a crack, high on the tibia,'' said Broncos head coach John Fox following Tuesday night's practice. "They're saying four to eight weeks. It could have been ligament damage and all that kind of stuff, it was not."
The third-year linebacker, who led the Broncos with 124 tackles last season, is one of the most important players in defensive coordinator Jack Del Rio's scheme.
Trevathan, who plays on the weak side in the base defense, is a three-down player at the position with enough strength to fill gaps in the run defense and enough speed and athleticism to work downfield in pass coverage.
"He's obviously a vital part of our defense," Fox said.
Earlier in training camp, Trevathan said he hoped to break through as a potential Pro Bowl player in the team's scheme.
"I just grind every day to be whatever I can be," he said then. "I set goals that are way high because I believe in what I can do. I just want to show it."
Fox said that Brandon Marshall, who was on the Broncos' practice squad for most of 2013, as well as rookies Lamin Barrow and Corey Nelson, would get some looks in the coming weeks as the Broncos try to fill in for Trevathan. Marshall took Trevathan's snaps in the defense Tuesday.
"You hate to see it," quarterback Peyton Manning said. "Danny is an important part of our football team."
Trevathan had a scare in the week before last season's game against the Dallas Cowboys on Oct. 6. He was carted off the field during practice after suffering a knee injury, but an MRI revealed no significant damage and he played against the Cowboys.