"It's pretty obvious we think Cody is playing a little bit better right now and deserves (to) start," Mike Riley said in a video posted on the Oregonian's web site.
Riley said Mannion does not have any physical limitations.
Mannion, returning from a torn meniscus on Saturday against UW, threw interceptions on consecutive possessions early in the fourth quarter after Oregon State had rallied from a 10-0 deficit to pull even. The last of Mannion's four picks was converted into points by Washington for a 17-10 lead with 8:19 remaining.
Cody Vaz entered the game and threw a 29-yard TD pass to Connor Hamlett with 4:58 left to pull the Beavers even. But the Huskies got a go-ahead field goal and time ran out on the Beavs.
Mannion has thrown seven interceptoins in his last two games.
"I'm just going to keep taking care of what I can control and work hard every day," Mannion told the Gazette-Times. "I'm going to support Cody any way I can. I told him I'm going to support him 100 percent. It's a disappointment, obviously, but I'm going to continue to work hard and become a better player."