Team owner Jerry Jones told reporters at a media party in Malibu, California, on Monday night that Scandrick tested positive for performance-enhancing drugs and lost an appeal on the ban.
Scandrick will be eligible to return Oct. 5 against Houston. He will miss the opener against San Francisco, and his final game on the sidelines will be against Drew Brees and the New Orleans Saints, who set an NFL record with 40 first downs against Dallas last year.
The Cowboys are practicing with Oakland on Tuesday and Wednesday at training camp in Oxnard. They break camp Friday.
Scandrick, who is going into his seventh season, became a starter last year when former top 10 pick Morris Claiborne struggled. Now Claiborne is battling tendinitis in his right knee and missed the preseason opener for the third time in his three seasons in Dallas.
Brandon Carr, the other starter, missed the first two weeks of camp to be with his mother in the final days of her battle against breast cancer. He might only play one preseason game but is expected to be ready for the regular season.
The Cowboys had to sign two defensive backs just to get through the preseason opener against the Chargers last week. They're likely to keep scouring the waiver wire, perhaps even looking for a veteran to give them depth while Scandrick is out.
B.W. Webb, a fourth-round pick last year, hasn't shown that he's ready to be a starter. Terrance Mitchell, a rookie seventh-round pick out of Oregon, could get a longer look while Scandrick is sidelined.