Porter has been sidelined by a hamstring injury that has limited him to only one appearance this year, against the New York Giants on Sept. 25.
Last season with the Raiders, the first season in which Porter played 16 games, he recorded 53 tackles, 1.5 sacks, two interceptions and one touchdown.
Washington had hoped he would cover the slot this year.
Washington coach Jay Gruden was more optimistic about tight end Jordan Reed. Reed, who has not taken the field since injuring his hamstring in Washington's Week 1 match-up with the Houston Texans, could play on Monday, Gruden said.
"We're going to make sure he gets another full day of rest and we're going to evaluate him again," said Gruden.
"There is a chance, we'll see tomorrow but today we didn't want anything worse to happen. We'll look at him tomorrow, give him a nice workout, and go from there."
In addition to Porter, quarterback Robert Griffin II is out. Defensive end Kedric Golston and linebacker Akeem Jordan are listed as questionable. Receiver DeSean Jackson and linebacker Brian Orakpo are both probable.
Washington is 1-3 this year and currently in fourth place in the NFC East. The team faces the Seahawks at home on Monday night.