Gregory didn't record a tackle prior to exiting. He was appearing in his first regular-season game since the final week of the 2016 campaign.
Gregory missed the entire 2017 season while serving a yearlong suspension for a violation of the NFL's substance-abuse policy. He was formally reinstated in July, which allowed him to rejoin the Cowboys for preseason practices and games.
However, ESPN's Adam Schefter reported Sunday that Gregory relapsed in August, which could result in another suspension.
Schefter added it's unclear whether the NFL will punish Gregory, writing "the league still is studying the case and factoring in the steps that Gregory has taken over the past month since his setback."
The concussion adds another question mark regarding Gregory's availability for Dallas' Week 2 matchup at home with the New York Giants.