The Redskins announced Saturday night that fellow rookie Kirk Cousins will make his first career start as Redskins (7-6) try to run their winning streak to five games and keep the pressure on the first-place New York Giants in the NFC East.
Griffin tweeted: "The decision was not my own... But I will be there for my team!!! In every way I possibly can".
The Redskins did not say whether Griffin will be active for the game, or whether veteran Rex Grossman, who has been an inactive third-string quarterback for every game this season, will serve as the primary backup to Cousins.
Griffin, who is tied with New England's Tom Brady for the NFL lead in passer rating and whose popularity has soared in a few short months as a 22-year-old newcomer to the league, was injured when he was hit by defensive tackle Haloti Ngata late in regulation during last week's 31-28 overtime victory over the Baltimore Ravens.
An MRI revealed a mild sprain of the lateral collateral ligament. The No. 2 overall draft pick and last year's Heisman Trophy winner was limited in practice all week, favoring the leg heavily on Wednesday but moving considerably better Thursday and Friday.
But offensive coordinator Kyle Shanahan said during the week that a decision on Griffin will rely more on the stability of the knee and the risk of further injury rather than how well the quarterback can move in practice, and head coach Mike Shanahan indicated the final call would hinge on an doctor's re-examination of the knee on Saturday.
The results of the examination apparently didn't go RG3's way.
Griffin has thrown 18 touchdown passes and only four interceptions this season and has a 104.2 rating. He has also rushed for 748 yards, an NFL record for a rookie quarterback.
Cousins was drafted in the fourth round from Michigan State, a surprise selection given that the Redskins had already selected Griffin. He made his first appearance after Griffin suffered a concussion against the Atlanta Falcons in Week 5 and threw one touchdown pass and two fourth-quarter interceptions in a 24-17 loss.
Then last week, Cousins went 2 for 2 in the Redskins' final drive of regulation after Griffin was injured, throwing an 11-yard touchdown pass to Pierre Garcon with 29 seconds left and then running in the game-tying 2-point conversion.
Cousins has completed 7 of 11 passes for 137 yards with two touchdowns and two interceptions in his two appearances. He has gained a reputation as a studious backup, benefiting from the fact that he and Griffin were brought along together in an offseason program and training camp specifically designed by Redskins coaches for quarterbacks new to the pros.
The Redskins were 3-6 before their bye week, but now they are one game behind the Giants and also one game behind the front-runners for the NFC's two wild card berths. Griffin has led the comeback, throwing for 10 touchdowns and only one interception during the four-game winning streak.