Barnard has been struggling with an ongoing groin problem and was ruled out of Saturday's 2-0 home defeat by Brentford.
"He's had two little things going wrong in the groin area," said Adkins.
"We were hoping to get one settled and the medical team have tried to do that but we've had to go down another pathway.
"It's frustrating but it's a fact of life.
"He might be out for a couple of weeks but we're hoping he'll be figuring during the Christmas period."