CORVALLIS, Ore. -- Cal's season turned from maddening to desperate Saturday, as starting quarterback Kevin Riley was lost with a potential season-ending injury to his left knee during the Bears' unsightly 35-7 loss at Oregon State.

The laundry list of what went wrong in yet another road dud is extensive, but all of that was overshadowed by the injury to Riley, who was taken down awkwardly by Beavers defensive tackle Brennan Olander on the Bears' second possession of the game. Riley was taken to the locker room for preliminary tests, but coach Jeff Tedford said the severity of the injury won't be known until Riley undergoes an MRI, possibly as early as today.

That being said, Tedford acknowledged that the injury was serious, saying, "Right now, it looks a little bit more serious than just your everyday sprained knee."

With only four games remaining, it seems a strong possibility Riley's season and Cal career are over. Riley watched the second half on crutches and wearing a knee brace. He declined to comment after the game.

"He was in a lot of pain," Tedford said. "He came off the field with tears in his eyes, and I'm sure that has a lot to do with his disappointment. He felt like it was pretty significant. Kevin is a tough guy. It's unfortunate."

Riley was replaced by junior Brock Mansion, who just two weeks ago moved his way from No. 3 to No. 2 on the depth chart. Mansion was understandably inconsistent, completing 14 of 24 passes for 138 yards and a touchdown, but most of those stats were padded on the Bears' last possession when the game had long been decided. Tight end Anthony Miller ended up catching a 45-yard scoring pass from Mansion with 20 seconds left as Cal (4-4, 2-3 Pac-10) avoided getting shut out for the first time since 1999.

"I felt pretty prepared," Mansion said. "You always prepare like you're the guy. Kev gets the majority of reps during the week but I take every single mental rep. It's almost as good as getting the physical reps. I felt like I was prepared."

When Mansion entered the game, it marked the first time a Cal quarterback other than Riley had taken a meaningful snap in a game since the 2008 Emerald Bowl, when Nate Longshore was the starter. Mansion had received some mop-up duty during Cal blowouts during the past few seasons. The Bears trailed 7-0 with seven minutes left in the first quarter when Riley went down.

"I was in shock," Mansion said. "I was like, 'I guess I'm going in.' After the first series, I was fine."

Tedford admitted the Bears shrunk the playbook when Mansion entered the game, but it might not have mattered how diverse the offense was. Cal was dominated at the line of scrimmage, providing negligible pass protection while opening up minimal holes for tailback Shane Vereen.

Cal's offensive line was also called for three false starts, two by right tackle Donovan Edwards. That was part of an overall sloppy performance by the Bears, who were penalized 12 times for 103 yards.

Cal amassed just 206 yards of offense, 93 coming on the game's final drive. Tedford said the Bears could make some changes along the offensive line.

"We definitely may," he said. "We're going to watch it on tape and find out what the issues are. We have to improve."

While the Bears were continually going three-and-out after consistently putting themselves in long yardage situations because of penalties, the Beavers (4-3, 3-1) sliced up Cal's defense time and time again. Star tailback Jacquizz Rodgers rushed for 119 yards and three touchdowns and threw an 11-yard touchdown pass to tight end Colby Prince out of the Wildcat formation.

The Beavers scored touchdowns on each of their first four possessions and didn't punt until there were 12 seconds left in the first half. By that point, Bryan Anger had punted five times for Cal.

"We have a pretty good defense. It's just consistency that needs to keep showing up," Cal defensive end Cameron Jordan said. "It wasn't so much what they did but what we didn't do. We'll have to look within ourselves and find ways to get better."

It didn't appear to be Cal's day from the outset. On the game's second play from scrimmage, Riley was forced to call timeout after the Bears had confusion lining up. Penalties and sacks forced Cal into tough third-down situations. Nine of the Bears' first 10 third downs were of double-digit yards.

"It's a bad combination when you're getting negative yards because of penalties and they're moving the ball very well and taking advantage of all the opportunities," Tedford said.

Riley obviously is out this week against Washington State, and there seems a good chance he won't be back. Tedford said Mansion will enter the week as the starter rather than have him compete with No. 3 quarterback Beau Sweeney for the job. Sweeney spent most of last year and the first six games of this season as the backup.

"Right now, it'd be Brock," Tedford said. "It was important to get Brock trying to feel comfortable. But all the way around, we didn't protect the passer very well. We didn't block very well. (There were) too many penalties. It was just a poor effort."

next saturday's game
Cal (4-4) at Washington State (1-8),
1 p.m. CSNCA