Connor Halliday's struggles hurt Washington State's upset bid on Saturday.
Halliday, the Cougars' sophomore quarterback, was pulled early in the second half of a 19-6 loss to No. 14 Oregon State after throwing his third interception.
He was replaced by senior Jeff Tuel, who was 11 of 17 for 126 yards.
Halliday finished 9 of 20 for 81 yards. Tuel briefly replaced Halliday in the second quarter, but Leach went back to Halliday to start the second half.
After Halliday had a turnover on the second play, he was gone for the rest of the game.
''I thought he was going to settle down,'' coach Mike Leach said. ''He's going to be a great player, but he wasn't settled down this game.''
Tuel also struggled. He was sacked three times and threw an interception to Jordan Poyer that ended the Cougars' hopes. Poyer also picked off Halliday twice.
The Beavers (4-0, 3-0 Pac-12) needed the defensive effort, as the offense - and quarterback Sean Mannion - struggled from the beginning.