EUGENE, Ore. — First it was Nate Costa. Now it's Justin Roper. Injuries to quarterbacks have Oregon experimenting.

Junior college transfer Jeremiah Masoli and true freshman Chris Harper will be rotated on Saturday when the No. 17 Ducks host Boise State. Masoli will be the likely starter, although coach Mike Bellotti wasn't quite sure.

It has been an unsettled season for Oregon (3-0) at quarterback. Projected starter Costa injured his knee late in fall practice and needed surgery that will likely keep him out for the rest of the season.

Roper found out he'd be the team's starter in the week leading up to the season opener against Washington. He started the Ducks' first three games — although Masoli and Harper made appearances in each one — until he was injured in the first overtime of a 32-26 victory over Purdue last weekend.

Roper, who has thrown for 484 yards and two scores this season, will miss this weekend's game and probably Oregon's next against Washington State. The team hopes to have him back for top-ranked Southern Cal on Oct. 4.

Masoli said the most recent turn of events means he's stepping up his preparation.

"I've just been trying to hit the film room and meetings and take it much more seriously," he said. "Last week we were preparing hard because you're always one play away from playing, but this week, I'm taking it to another degree."

Masoli has completed 16 of 28 passes for 193 yards and two TDs. Harper has run for 102 yards and two touchdowns, while going 4-of-6 for 40 yards and a touchdown.

The uncertainty at quarterback will be a challenge for Boise State (2-0).

"They have their system, style of offense. And within that system, they'll try to play to those kids' strengths. Hopefully there will be enough tape out there that we can predict right what they're going to do," Broncos coach Chris Petersen said.

Boise State is 2-11 against the Pac-10, and 0-8 on the road against the conference. The Broncos are coming off of a 20-7 victory over Bowling Green.

Boise State and Oregon have never met, although there's some familiarity between the teams.

Petersen was wide receivers coach at Oregon from 1995-2000, and Boise State defensive coordinator Justin Wilcox played at Oregon from 1995-99. Oregon safety Patrick Chung even played against Boise State running back Ian Johnson in high school.

Johnson will be targeted by Oregon. He has 184 yards and three touchdowns on 27 carries this season.

"Oh no, he's not going to take the spotlight from us. This is our house. He's not going to take it, I promise," Oregon running back LeGarrette Blount said.

Blount and Jeremiah Johnson lead Oregon's running game. The Ducks are ranked fourth in the nation with an average of 323.3 yards per game. Oregon is also ranked fourth in total offense, with an average of more than 562 yards.

Blount has rushed for 285 yards and four touchdowns, while Jeremiah Johnson has run for 257 yards and two touchdowns — even though he was knocked out of Oregon's second game against Utah State with a shoulder injury.

Petersen said it's clear to his players what a threat Oregon can be.

"The main thing in my opinion is to make sure these guys don't get too much anxiety up and play our best football," he said. "We've got to work hard on focusing in and playing football just like we did the last couple of weeks, even though the environment is a little different."