Ducks starting quarterback sprains left knee against Purdue

Oregon starting QB Justin Roper is expected to miss two to four weeks with a sprained knee. AP Photo/Tom Strickland

WEST LAFAYETTE, Ind. -- Oregon starting quarterback Justin Roper likely will miss two to four weeks after spraining his left knee in Saturday's 32-26 win against Purdue.

Ducks coach Mike Bellotti expects to have a definitive prognosis on Roper by Monday but doesn't expect the sophomore to play next week against Boise State. Roper sustained the injury lunging for a first down in the first overtime. Freshman Chris Harper relieved him and led Oregon to the game-winning touchdown in the second overtime.

Roper was on crutches after the game and declined to speak with reporters. Oregon already has lost projected starter Nate Costa, who suffered a season-ending knee injury during preseason camp.

Bellotti will go with Harper and sophomore Jeremiah Masoli against Boise State. Freshman Darron Thomas also will be available.

"You have to have three quarterbacks and Darren will get into the rotation," Bellotti said. "It's farther on the depth chart than we would have liked to go, but that's the way it is."

Adam Rittenberg covers the Big Ten for ESPN.com. He can be reached at espnritt@gmail.com.