Britton, who missed the final nine games last season because of a shoulder injury, hurt his back two weeks ago. Coach Jack Del Rio said Britton had a procedure in North Carolina and will be "out a little while." Del Rio said Britton could be available for the Sept. 11 season opener.
Miller, who missed parts of the last two preseasons, has a sprained right knee but could return for the opener.
"I was very impressed the way the [Jaguars] football team looked tonight," said Falcons head coach Mike Smith told the Florida Times-Union. Smith was the Jaguars' defensive coordinator from 2003-07. "There's a lot of young physical guys, and I thought we had a very good session with these guys, but there's probably only about eight or nine guys on the roster that were here when I left."
"It's always fun to get out there and play a different team," Jacksonville quarterback Blaine Gabbert told the Times-Union. "It's a great change of pace. Couple starters from Mizzou, so it was fun going against those guys again in 7-on-7, team period. Overall, it's a great night."
Veteran David Garrard, who is trying to fend off a challenge from the rookie Gabbert for the quarterback job, will get the start Friday against Atlanta.
The Associated Press contributed to this story.