EUGENE — The season hadn’t even started for Oregon and already the Ducks were having to make adjustments.

Sophomore quarterback Nate Costa, projected as the starter for the No. 21 Ducks, re-injured his surgically repaired left knee last week during practice. He had additional surgery on it this week and is out for the season.

So it will be sophomore Justin Roper who starts when Oregon opens its season against Northwest rival Washington tonight at Autzen Stadium.

Roper was pressed into duty last season after Oregon lost quarterback Dennis Dixon, a Heisman hopeful who injured his knee just as the Ducks were making a run at a national title, as well as backup Brady Leaf.

Roper played in a total of four games last season, including a start in the Sun Bowl against South Florida, when he threw four touchdown passes in Oregon’s 56-21 victory.

Oregon coach Mike Bellotti said Roper and Costa were both working with the starters that last two weeks of fall camp, running the Ducks’ spread-option offense, so he’s confident that the changes won’t be too jarring.

“He (Roper) did great things in our bowl game, so I’m really excited to see if he can continue that kind of development,” Bellotti said.

Costa, because he had very limited live-game experience, was more of a question mark for Washington than Roper. Costa had appeared in just one game for Oregon, during his freshman season in 2006.

“It helps us a little bit, we know what to watch on film, we know who to watch on film. We can key his tendencies, his releases and who his main receiver is,” Huskies safety Nate Williams said. “It helps us out, hopefully it will work out on Saturday.”

It helps that Roper is already familiar with a big-game atmosphere, because he’ll see it against Washington.

The Ducks have won the last four games against Washington, their longest streak in the series since 1928-31. It is the 101st meeting between the two teams, dating back to 1900.

The Huskies return dual-threat quarterback Jake Locker, who accounted for 59 percent of Washington’s total offensive yards as a freshman in 2007.

Locker ran for 986 yards and 13 touchdowns last season, while throwing for 2,062 yards passing and 14 TDs. But his talent wasn’t enough to carry Washington, which went 4-9 overall and 2-7 in the Pac-10.

It was Washington’s fourth-straight losing season. The Huskies are 6-20 in the Pac-10 Conference under Tyrone Willingham, and winless against Oregon teams.

Willingham said he expected to start 10 freshmen or sophomores today, including tailback Chris Polk and defensive tackle Senio Kelemete, both true freshmen.

The university believes Polk will be the first true freshman to start a season opener at running back for the Huskies since freshmen became eligible in the 1970s.

Redshirt freshmen Alvin Logan (wide receiver) and Quinton Richardson (cornerback) will also get their first starts against the Ducks.

Three-quarters of the defensive line is new and looking to improve on last season when Washington’s defense allowed 446 yards per game.

Oregon went 8-1 last season before the loss of Dixon, and finished 9-4. The Ducks return six starters on offense, including center Max Unger, receiver Jaison Williams and tailback Jeremiah Johnson.

Johnson is the unquestioned starter after three years of splitting carries with Jonathan Stewart. In 27 career games, the senior has 1,135 yards and 17 touchdowns on just 181 carries. He also has 28 receptions for 228 yards and one touchdown.

On defense, Oregon returns seven starters including defensive end Nick Reed and safety Patrick Chung, who led the Ducks and was third in the conference last season with 117 tackles.

Roper’s backup is junior college transfer Jeremiah Masoli, who led City College of San Francisco to the junior college national championship last season while throwing for 3,592 yards and 30 touchdowns.

Oregon beat Washington 55-34 in Seattle last season.