Golden Bears coach Jeff Tedford said Mansion, a redshirt junior who has played sporadically over the last two seasons, will start ahead of sophomore Beau Sweeney in Cal's game at Washington State this week. Sweeney had been Riley's backup until two weeks ago.
Riley was hurt early in Saturday's 35-7 loss to Oregon State when he was hit in the left knee by Beavers defensive tackle Brennan Olander. Riley's knee bent backward and the senior quarterback immediately fell to the turf clutching his leg. MRI tests later revealed he suffered a sprained ACL and strained MCL and ACL. Riley also partially tore the calf muscle connected to his knee.
Only a year ago Mansion was buried on Cal's depth chart behind Riley and Sweeney. He then began this season as the Bears' third quarterback before being elevated to the backup job two weeks ago.
Now he's set to make his first start since high school.
"It has tested my patience but you have to keep grinding and grinding," Mansion said after practice Tuesday. "The harder you work for a longer period of time, the better it's going to pay off for you at the end."
Running back Shane Vereen noted the Bears had to shrink their playbook when Mansion replaced Riley against Oregon State but said that won't be the case this week.
"We know his capabilities, we know the quarterback he can be and I think he's going to step up to the plate and deliver for us," said Vereen, the Pac-10's fourth-leading rusher. "[Riley] has always been a tough player for us, and to see him go down was kind of tough. But at the same time we have to rally behind Brock. I think he'll be able to cut it loose. We'll be fine."
Mansion steps into a tough situation. The Bears are unbeaten at home but haven't won a road game all year.
"Since [Riley] has been down I've been trying to be a sponge and absorb as much as I can, little mechanical things," Mansion said. "I can't really describe it but you just feel that much more comfortable understanding the timing and what we want. I want to cut it loose, play football and put some points on the board."
For Riley, the injury marks the end of an up-and-down year. The Bears' unquestioned starter the last two seasons, he led Cal to a pair of early blowout wins but came under fire recently when the team lost its first three road games, including a 48-14 blowout at USC on Oct. 16.
He rebounded to lead the Bears past Arizona State before getting injured against Oregon State.
Riley's injury was the latest setback but won't require surgery.
"I don't need surgery, which is a positive," Riley said. "But it's going to be like three months."
Tedford said Riley will remain with the team through the remainder of the season.
"He mentioned to me yesterday that he's going to be here for Brock," Tedford said. "Once he feels like he can be around and get around OK, I'm sure he'll be here every day giving everybody encouragement."
As for starting a new quarterback in Mansion, Tedford expects the Bears to go forward with their full playbook.
"It's not like we pulled him off the scout team and it's something new," Tedford said. "I thought he had a great practice today. He let it rip and was very sharp. Any time you go into a game and it's your first start there's going to be some anxiety. But Brock has a very good mindset, he's very confident and he'll be prepared to play."