Gabriel and Braunecker both self-reported their concussions after the Bears' win against the Giants on Sunday and didn't join the team for a light walk-through at Halas Hall on Monday afternoon.
With the Bears already thin at tight end, the team would turn to rookie Jesper Horsted as its starting "U" tight end. Horsted caught his first career pass Sunday.
"He definitely has talent," Nagy said of Horsted. "It's going to take him some time to learn the position. We're going to be patient with that."
The Bears also practiced Monday without right tackle Bobby Massie (ankle), linebacker Danny Trevathan (elbow) and special teams standout Sherrick McManis (groin). Their statuses for Thursday are also in doubt.
The Lions listed quarterback Jeff Driskel (hamstring) as limited, though though that was an estimate as Detroit didn't hold a practice. David Blough would be the Lions' quarterback if Driskel is unable to play, though the Bears don't see that as likely.
"We're planning on Driskel going," Bears defensive coordinator Chuck Pagano said.
Kickoff between the Bears (5-6) and Lions (3-7-1) comes Thursday at 11:30 a.m. CT at Ford Field in Detroit.