"Christian's going to out for a while with a back injury," said new City boss Coppell.
"He's had it before but I think it was exasperated by playing for Wales when he perhaps shouldn't have done."
Coppell added: "With back problems you have to just let them heal at their own pace. I had a back problem last year and it took time to shake off."
Ribeiro's latest injury will be a further blow for the 20-year-old who has already spent a year out of the game with a cruciate knee ligament injury.
Then, as he tried to fight his way back into the Bristol City team last season under former boss Gary Johnson, he saw a loan spell at Colchester United cut short by a groin strain.
Coppell will also be without Ribeiro's Wales Under-21 defensive team-mate James Wilson who has picked up a knee injury.
"James has a knee problem and tendon issues are also difficult to shift," he said.
"We're making sure that once it goes away, it goes away for good. We don't want to force anything and make the injury reoccur.
"They are both good players with bright futures so we're erring on the side of caution."
Source: BBC Sport
Source: BBC Sport