McBath broke his right forearm in the Broncos' 28-16 loss at Indianapolis on Sunday. He was hurt in the fourth quarter while playing safety in place of starter Renaldo Hill, who had left the game with an ankle injury.
McBath was placed on injured reserve Monday, ending his season. Safety Vernon Fox, one of Denver's final cuts during camp, was signed to replace him.
McBath is the second player on the AFC wild-card leaders to go on IR in the last week, joining standout right tackle Ryan Harris (toe).
A second-round draft pick out of Texas Tech, McBath was the most productive of coach Josh McDaniels' inaugural rookie class aside from tailback Knowshon Moreno, who is splitting snaps with Correll Buckhalter out of the backfield.
Although he's the team's top reserve safety, filling in at times this season for both Hill and Brian Dawkins, McBath made his biggest mark in the kicking game.
"He's been a big part of our special teams," McDaniels said. "He's the personal protecter on the punt team, which is no small chore. He handles all the communication there. He's our leading tackler, very unselfish, plays great on all of the teams. He has been a great addition in that regard and he will be missed in that sense."
Fox is an eighth-year safety who played in Denver last year and had been out of football since his Sept. 5 release.
"Vernon Fox will have an opportunity to compete to fill in right away because he was one of our personal protecters, as well," McDaniels said. "He shared that responsibility all the way through the preseason. I think Vernon can do that right away [and] which I have confidence in him. He's a very bright person, understands that role very well.
"So, if he can do that for us, that would help us not to have to juggle a bunch of balls in the air and move everybody around."
With Hill's status uncertain, another rookie, David Bruton, who missed the Indy game with a thigh injury, could see his first significant playing time Sunday when the Broncos host the Oakland Raiders.