Shanahan said Tuesday the suspension was issued "a while back" from the league office.
The coach did not say why Buchanon was suspended or give any other details.
Shanahan revealed the news the day after Buchanon announced on Twitter that he was re-signing with the Redskins.
"I'm excited to return to the Washington Redskins and continue to be part of this great organization," Buchanon said in a statement released Tuesday by his publicist. "I look forward to putting the suspension behind me, returning to the field and helping my team."
Buchanon, 30, was mostly a nickel back for Washington last year. He started five games and had two interceptions.
Copyright 2011 by The Associated Press