Earlier this week, the NFL handed Dareus a four-game suspension by the NFL for violating the league's substance-abuse policy. He will have to sit out a difficult opening stretch for the Bills -- at the Baltimore Ravens, home against the New York Jets and Arizona Cardinals, and then at the New England Patriots.
"After careful consideration of recent events within my life, I have made the decision to enter a rehabilitation facility," Dareus said in a statement issued by the Bills. "I feel this is the best decision for me, my family and my teammates as I move forward with my career. I appreciate the support of my teammates and the Bills organization through this process."
Dareus sat out the 2015 season opener against Indianapolis as punishment for his initial substance-abuse offense. This second one was the result of a missed test which, according to the NFL, is the same as a positive one. NFL Media's Ian Rapoport reported this week that the suspension involves marijuana use.
The Bills just gave Dareus a $103 million contract extension last year which included $60 million in guaranteed money. Dareus will not lose any of his future guaranteed money because of this incident. The suspension will cost Dareus about $3.3 million.
"We are fully supportive of Marcell and this decision to take the time he needs to seek treatment," the Bills said in a statement.
Later in the day, general manager Doug Whaley reiterated the team's support of Dareus.
"He's family and we're being supportive of him in this step in his life," Whaley said. "I told (the media) we wouldn't give up on him and again, we whole-heartedly support this effort."
Dareus is with the team but did not play in Saturday's preseason game against the New York Giants.
There is no timetable for Dareus beyond the suspension.
Dareus' absence puts the defense in a real bind, especially considering the Bills are already playing without their top two draft picks, both of whom were projected as starters -- edge rusher Shaq Lawson and inside linebacker Reggie Ragland.