Eugene, OR (Sports Network) - Oregon quarterback Justin Roper will be out two to four weeks because of a partially torn ligament in his left knee. Ducks coach Mike Bellotti said Roper injured his knee late in a 32-26 overtime victory at Purdue on Saturday and that a preliminary diagnosis indicated a partial tear of Roper's medial collateral ligament. Further tests were scheduled for Monday to determine the extent of the damage. Roper was picked to start for 17th-ranked Oregon (3-0) the week before the team's opener against Washington when projected starter Nate Costa injured his left knee in practice. Costa had surgery and is expected to miss the season. In three games this season, Roper went 40-for-77 for 484 yards along with two touchdowns and four interceptions. He also added 88 yards on the ground on 11 carries with a score. The Ducks host Boise State on Saturday, and the team is expected to use backup Jeremiah Masoli and true freshman Chris Harper at quarterback. Bellotti did not specify who would start Saturday's game, but he did say that both would see action in the contest.