Bristol City defender Christian Ribeiro could miss the start of the Championship campaign through injury.
The Wales international has struggled with a long-standing back problem and is expected to be ruled out for several weeks.
Manager Steve Coppell has a number of injury concerns ahead of the pre-season tour to Sweden but has insisted he will not rush players back into the first team.
Speaking to reporters, he said: "Christian's injury is giving cause for concern and he could be out for a number of weeks. Backs are funny things and you have to be so careful with them. We can't afford to rush him back and then run the risk of him breaking down again.
"We'll monitor his situation closely and give him all the time he needs to make a full recovery. At the moment, it's difficult to say how long that might take.
"I'm not too worried about Louis Carey and he should be back with us in a few days. But Christian Ribeiro and James Wilson look as though they're going to be longer-term, while we have to be very careful with Jamie McAllister.
"He's raring to go and would be back in training today if he had his way, but his injury was more serious than first thought and we have to look after it.
"My priority is to have everyone fit for the first game against Millwall, on August 7, not to push them too hard too soon and run them into the ground."