Davis, who has 36 receptions for 517 yards and two touchdowns this season, had been listed as questionable because of an ankle injury. The Redskins already are without tight end Chris Cooley for the season because of a knee injury.
Redskins safety Oshiomogho Atogwe, though, is inactive with toe and knee injuries. He played last week but missed the game before that.
Niners defensive tackle Ray McDonald is inactive with a hamstring injury, the first time he's missed a game since 2008.
The Associated Press contributed to this report.