BOISE, Idaho -- Boise State running back D.J. Harper likely will miss the rest of the season with a torn anterior cruciate ligament in his left knee.
Harper, the team's second leading rusher, was injured in Friday's 51-34 victory over Fresno State when he was smothered in the backfield late in the game.
He is one of two tailbacks who share carries for the No. 8 Broncos (3-0) and rushed for 292 yards and three touchdowns on 44 carries.
Coach Chris Petersen said Harper was undergoing tests Monday, and team physicians are all but certain Harper will need surgery.
"It's hard on all of us, it's hard on the offense and it's hard on D.J.," Petersen said Monday. "He's a heck of a back, and we were just starting to see the best of him. He'll be missed, no question about it."
Harper rushing for 278 yards and four touchdowns last season as part of a crowded backfield that included Jeremy Avery, Doug Martin and senior Ian Johnson, the all-time rushing touchdown leader in the Western Athletic Conference.
With Johnson gone and Martin playing defense, Harper worked hard in the offseason to take advantage of a more expanded role. In the first three games, he and Avery were hard to stop, helping the Broncos amass 590 yards on the ground.
Harper ran for 88 yards against Oregon, added 89 yards against Miami, Ohio, and had 107 yards, including a 60-yard touchdown run, before leaving in the second half against Fresno State.
Martin, a special teams ace who also has lined up as a nickel back on defense this season, will replace him. Petersen said he expects the transition for Martin and the offense will be seamless. The first test comes Saturday when the Broncos travel to Bowling Green (1-2).
"I hope it [running game] doesn't change," Petersen said. "I hope that Doug and Jeremy are now the one-two punch we've been talking about all along. Hat's off to Doug Martin. We move him to defense and now we need him back on offense and the guy doesn't bat an eye."
Petersen said Harper should be able to redshirt this season, giving him two more seasons of eligibility.