The players will be able to participate in training camp and OTAs with their respective teams, but will miss the first four games of the season.
"I apologize for letting down my family, teammates, this organization and the fans," Richardson said in a statement. "However, words aren't enough. This is something that can only be addressed by how I handle myself from this point on. I don't want this to take away from what the team is trying to accomplish. While I won't be there at the start of the regular season, I will do whatever I can to support my teammates until I'm able to return to the field."
McClain released a similar statement through the NFL Players Assn., apologizing for his actions.
"I apologize to my family, the Cowboys organization, my teammates and Cowboys fans for my mistake. I will not break the rules of my profession in the future, and I regret my error. I look forward to returning to the field on week 5, when I hope to help my team beat the
Richardson, 24, was the 2013 NFL defensive rookie of the year and had eight sacks last season with 64 tackles and one forced fumble in his second season. His absence could mean more time on the field for former USC defensive tackle Leonard Williams, who was selected with the No. 6 overall pick in the draft.
McClain, 25, came out of retirement last year to play for the Cowboys after Dallas lost starting linebacker
Lee is back but the Cowboys defense will also be without defensive end
Gates, 35, admitted in a statement released by the Chargers that he had indeed tested positive for a banned substance.
"In my 12 years in the NFL, I have taken tremendous pride in upholding the integrity of the NFL shield and all that it entails, Gates said. "I have taken extreme care of my body with a holistic approach and never taken any substance that was illegal or banned by the NFL.
"In an effort to recover from a long season and although I was unaware at the time, I regret to confirm that I tested positive for a substance that is currently on the NFL banned substance list." he continued. " As an NFL veteran and team leader, I should have done my due diligence to ensure that what I was taking for recovery was within the NFL guidelines. I have always believed that ignorance is no excuse when it comes to these issues, and I take full responsibility for my actions."
Gates caught 89 passes for 821 yards and 12 touchdowns last season.