The Cowboys put Ben Bass on injured reserve Friday.
He suffered a dislocated shoulder in practice on Thursday, and coach Jason Garrett initially said the team did not believe it was season-ending.
But a separated shoulder, if it requires surgery, can mean six months of recovery.
Bass is the second Cowboys defensive end lost for the season. They lost Tyrone Crawford to an Achilles injury in the first practice of training camp. Since then, they have placed defensive tackle Jay Ratliff on the physically unable to perform list, meaning he will miss at least the first six weeks.
The Cowboys acquired two defensive ends in the past week. They traded for Edgar Jones of Kansas City and Caesar Rayford of Indianapolis. Additionally, they will bring back defensive end Jerome Long, who was released at the end of training camp.
-- Carlos Mendez