The punishment is on top of the four-game suspension Gregory was already serving.
He had appealed the punishment, but the Star-Telegram's Clarence E. Hill Jr. reported Sept. 19 the second-year pass-rusher withdrew the appeal.
According to Bleacher Report's Jason Cole, Gregory hoped to receive a lighter suspension that would have allowed him to return earlier:
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A source told Hill the 10-game ban resulted from a fifth violation of the NFL's substance abuse policy:
Gregory has had four policy violations and/or failed four tests since February 2015. The first one was at the NFL Scouting Combine, which landed him in the substance abuse program. A second resulted in a two-game fine. A third one resulted in a four-game fine and the fourth one left him with a four-game suspension.
On July 26, the Cowboys announced Gregory had entered a rehabilitation facility.
In a Sept. 13 interview with 105.3 FM The Fan (via David Helman of the Cowboys' official website), team owner Jerry Jones announced Gregory had been discharged and returned to the team.
During a press conference Sept. 21, Cowboys head coach Jason Garrett praised Gregory's work ethic since his return, courtesy of the team's Twitter account:
The 10-game suspension may in effect end Gregory's 2016 campaign. He's eligible to return in Week 16, so he could see the field again between now and the end of the regular season. However, Gregory can't practice with the team until Dec. 19, according to ESPN.com's Todd Archer.
That doesn't leave Gregory with much time to get ready for two potentially critical games for the Cowboys. If Dallas is in the playoff hunt, Garrett may be wary of inserting a player who hasn't seen meaningful action since the end of last season.