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.
Del Rio said linebacker Clint Session and running back Rashad Jennings, both of whom sustained concussions in the Jaguars' preseason opener at New England, will be held out of Friday's home game against Atlanta.
"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.