Portis left the game with 3:20 remaining in the first quarter after a helmet-to-helmet hit from DeCoud. The Redskins said Portis suffered a head injury and would not return.
There was no penalty on the hit.
Portis was helped to his feet after being checked by the medical staff on the field. He appeared shaky at first, but then walked off the field with a towel over his head.
Washington quarterback Jason Campbell left the game with an ankle injury in the fourth quarter but returned for the next possession. Campbell was hurt when tackled from behind by Jonathan Babineaux. The Redskins said he was questionable to return.
Campbell walked slowly off the field and received medical attention on the sideline.
Redskins offensive tackle Mike Williams left with an ankle injury on the same play.
It was Campbell's second injury of the game. He left with a chest problem after he was sacked for the fifth time in the first half. He returned for the start of the second half.
Todd Collins replaced Campbell after each injury.
Falcons receiver Brian Finneran left the game in the first quarter with a knee injury and did not return. There were no immediate details on the severity of the injury.