Shanahan said the suspension was issued "a while back" from the NFL 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."
The NFL had no comment on the suspension. The league's drug-testing programs were not in operation during the recent lockout, and no disciplinary action was issued during that time, so it appears the Redskins would have been notified of Buchanon's sanction before the work stoppage began in March.
Buchanon, 30, was mostly a nickelback last year. He started five games, mostly while Carlos Rogers was injured, and had two interceptions.
He also has played with Oakland, Houston, Tampa Bay and Detroit in nine NFL seasons.