In July, McGary was suspended the first five games of the season because of a drug violation, and with a second suspension, he will now miss the opening 15 games.
According to The Vertical, this most recent suspension is not another failed drug test but "failure to live up to procedural guidelines set forth in the program."
McGary, who was selected 21st in the NBA draft in 2014, now has a shaky future with the Thunder. McGary left the team last season for a few weeks because of personal reasons, returning during the postseason.
The 24-year-old forward showed promise in his limited time on the floor, flashing potential as a skilled big man who can run the floor and pass, but injuries, difficulty staying in shape and other off-the-floor issues have kept him from making a real impact.
McGary's contract is guaranteed for next season ($1.52 million), but the Thunder could trade him or buy him out of his remaining deal. It's unlikely McGary will play another game for the Thunder.
McGary declared for the NBA draft after testing positive for marijuana while at Michigan following his sophomore season. Had he not entered the draft, he would've been suspended by the NCAA for his entire junior season.
After his five-game suspension was announced in July, McGary issued a statement through the team: "I clearly made a mistake, and for that, I apologize to the organization, my teammates, and our fans. I realize there are only so many opportunities given to you and I need to make sure that, as a person and as a player, I take accountability and ensure that this does not happen again."