Alexander signed a one-year deal with the idea of being around until Ladell Betts recovers from a knee injury. Betts is expected to miss around three weeks with the injury. Alexander will back up Clinton Portis during Betts' absence.
"I'm in great shape. I'm so excited to play. I'm finally 100 percent healthy," Alexander told The Associated Press last month. "I really don't care what part of the game I'm in, whether it's a complementary role or whatever."
Alexander, released by the Seahawks in the offseason, had visited the Saints, Bengals and Lions before landing a job in Washington. The fit is natural because Alexander worked with Redskins head coach Jim Zorn and running backs coach Stump Mitchell in Seattle.
Alexander, 5-foot-11 and 228 pounds, rushed for 9,428 yards in eight seasons with the Seahawks.
The Redskins also cut sixth-round draft pick Durant Brooks, who ranks last in the league in punting, and replaced him with former Seattle punter Plackemeier. Plackemeier won a competition among four free agents flown in to audition at Redskins Park on Tuesday morning.
Brooks was technically listed as "waived-injured" because he has a nagging hip flexor and quad strain in his kicking leg, but Zorn made it clear Monday that Brooks' job was in jeopardy because of performance.
Reed Doughty, the Redskins' starting strong safety at the end of training camp, had his season end when he was placed on injured reserve. Doughty has been experiencing a nerve problem that started in his back and recently caused numbness in one foot. Doughty had already lost his starting job to rookie Chris Horton.
To replace Doughty, the Redskins added Green, yet another former Seattle player. Little-used safety Justin Hamilton was also released.
Plackemeier was also cut by the Seahawks this year, let go after he had a pair of 22-yard punts in the season-opening loss to the Buffalo Bills. He was Seattle's punter for two full seasons and has a career average of 42.3 yards with a 35.5 net.
Zorn spent the past seven years as the Seahawks' quarterbacks coach, so he knows Alexander and Plackemeier well. Green comes with an extra bonus: He was not only with Zorn in Seattle, but he also played under Redskins defensive coordinator Greg Blache when Blache held the same job with the Chicago Bears.
Green played six years with the Bears and two with the Seahawks, but his time in Seattle was marred by a severe foot injury. He was cut at the start of training camp in July.
John Clayton is a senior writer for ESPN.com. Information from ESPN.com's Mike Sando and The Associated Press was used in this report.