FOX Sports Exclusive
Injured RG3 won't start vs. Browns
As FOX Sports reported on Friday, Cousins took snaps with the first team all week. Griffin, the league's top-rated passer, took part in individual drills while the media were on hand, but then was shut down when reporters left.
Shortly after the announcement Saturday night Griffin tweeted: "The decision was not my own... But I will be there for my team!!! In every way I possibly can."
It will be the first missed start for the 22-year-old former Heisman Trophy winner out of Baylor. Griffin, the No. 2 pick in the 2012 NFL Draft, has thrown for 18 scores this season and rushed for six more.
The Redskins, 7-6, have won four in a row and enter Week 15 one game out of an NFC wild-card spot. The Browns are 5-8.
Cousins, a fourth-round pick out of Michigan State, finished last week's game after Griffin was injured. He threw a touchdown pass in the final minute and ran in the game-tying 2-point conversion. He has appeared in two games this season, completing 7 of 11 passes for 137 yards with two touchdowns and two interceptions
The Associated Press contributed to this report