Buchanon has been troubled by neck and knee problems and appeared in only two games this season after being suspended for the first four games for violating the NFL's policy on performance-enhancing substances.
This was his second season with Washington and 10th in the league.
The Redskins replaced Buchanon by adding cornerback Brandyn Thompson to the 53-man roster Saturday. Thompson was a seventh-round draft pick in April and has played in two games.
Washington (3-4) hosts the NFC West-leading San Francisco 49ers (6-1) on Sunday.