EUGENE — Quarterback Nate Costa was expected to be the new face of the No. 21 Oregon football team, but a recent injury to his surgically repaired left knee has moved the Ducks’ most significant position battle back to where it started when fall camp opened three weeks ago.

Costa, the presumptive starter, got hurt during a closed practice Wednesday night when he tried to change directions during a scramble.

Costa finished the final 20 minutes of practice and then alerted members of the Oregon coaching staff that he felt some discomfort in the knee. He had reconstructive surgery on it last fall after tearing ligaments in an October practice.

The redshirt sophomore underwent testing Thursday and will wait to get a diagnosis after the team’s orthopedic surgeon returns to Eugene Sunday.

“It was a noncontact injury,” Oregon coach Mike Bellotti said. “I saw the play on film, and you can tell he does something to his knee.”

Costa appeared to have won the starting job during a camp battle with 2007 Sun Bowl hero Justin Roper and a trio of newcomers.

Now, Roper moves into the No. 1 spot and is considered the favorite to start the Ducks’ opener at Autzen Stadium on Aug. 30 against Pac-10 Conference rival Washington.

The 6-foot-6, 205-pound redshirt sophomore from Buford, Ga., was Oregon’s fifth-string quarterback at the start of the 2007 season. But after season-ending injuries to the Ducks’ top three quarterbacks, Roper made his first career start in the Sun Bowl and threw for 180 yards and four touchdowns in a 56-21 win against South Florida.

Roper came into fall camp expected to push Costa for the starting job. He outplayed Costa in the Ducks’ scrimmage on Aug. 16 when he completed 7 of 8 passes for 155 yards and three touchdowns.

“I felt like he was probably overall the most impressive quarterback today and think overall Justin made up some ground,” Bellotti said afterward.

Oregon had the Pac-10’s most potent offensive attack in 2007.

Led by Heisman Trophy hopeful Dennis Dixon at quarterback, 1,700-yard rusher Jonathan Stewart in the backfield and new offensive coordinator Chip Kelly’s potent spread-offense schemes, the Ducks averaged 38.2 points and 467.5 yards a game en route to a 9-4 record last fall and a 56-21 bowl win against South Florida.

Despite the loss of both Dixon and Stewart, Oregon returns six starters on offense, including one of the nation’s top centers in Max Unger, as well as receiver Jaison Williams and tailback Jeremiah Johnson. Johnson is coming back from a knee surgery that cost him the final six games of 2007.

Williams, a 6-5, 240-pound senior, has had a standout camp, an encouraging sign for the Ducks, who will be breaking in a pair of new starters in the receiving corps.

In the backfield, Johnson is expected to be the primary runner after three years of splitting carries with Stewart. In 27 career games, the senior from Los Angeles has 1,135 yards and 17 touchdowns on just 46 carries. He also has 28 receptions for 228 yards and one TD.

Johnson’s backup is expected to be LeGarrette Blount, a 6-2, 229-pound junior college transfer who was one of the top-ranked JC recruits in the nation.

Oregon returns seven starters on defense, including end Nick Reed, who led the Pac-10 in 2007 with 12 sacks and 22.5 tackles for a loss.

Hard-hitting senior Patrick Chung is regarded as one of the nation’s top safeties. He led the Ducks and was third in the conference last season with 117 tackles.

Junior cornerback Jairus Byrd led the Pac-10 with seven interceptions last season, while fellow junior cornerback Walter Thurmond III recorded five interceptions and was tied for the league lead with 18 passes broken up.

The Ducks’ nonconference schedule features games at Purdue and home against Boise State, while their Pac-10 slate will include road games against the Trojans, Sun Devils, Cal and Oregon State.