The redshirt sophomore broke a bone in his ankle, according to head coach Bryan Harsin.
SOU (2-1, 2-1 Frontier) scored 20-straight second quarter points to erase a 14-7 C of I lead, getting two long second half touchdowns to put the game away.
The Broncos (2-1), coming off a gut-wrenching loss to BYU a week ago, suffered another tough loss when starting quarterback Ryan Finley left in the first quarter with a right ankle injury and never returned.
More than half of the College of Idaho football team is from the Treasure Valley.
Ryan Finley has thrown 1 touchdown and 4 interceptions this season.
Tanner Mangum played one season at Timberline before transferring to Eagle.
The Broncos fill up a semi-truck trailer with around 7,000 pounds.
Tanner Mangum returned from a Mormon mission in Chile and a two-year hiatus from football just three months ago.
The move comes just one day after he was cut by the Detroit Lions.
No matter how many games Tanner Mangum plays for BYU, it will be hard for him to top what he did in his first one.
College of Idaho drops to 1-1 on the season.
The 23rd-ranked Broncos came up with enough plays to send Petersen home with a loss.
Bronco players describe him as a vocal leader.
Chris Petersen is adding secrecy to his anticipated return to Boise State.
Boise State and Washington to meet up on Friday.