On his first dropback of the game against the Detroit Lions on Sunday, he was sacked by linebacker Jarrad Davis and, in the process, lost his helmet. It's speculated that the impact of that hit specifically was enough to give Rodgers a concussion.
Rodgers remained in the game for two more series after that initial play but didn't have much to do with anything as the Packers failed to create any semblance of offensive productivity. He eventually was shown walking to the team's locker room with Dr. John Gray, one of the Packers' team physicians -- also a sign that Rodgers was being evaluated for a concussion.