Defender Liam Fontaine missed the home victory over Derby after sustaining an ankle injury in training last week.
He is set for at least a month on the sidelines on the advice of a specialist and
Millen said: "I'll be sitting down with our medical staff to discuss the way forward. It's not great news and we may have to get another opinion."
Fellow defender Louis Carey has had an injection in his troublesome calf problem. The City skipper has not played since suffering the injury at Leeds on November 13 and could miss the entire Christmas schedule.
Marlon Jackson has returned from his loan spell at Aldershot but the striker is nursing a hamstring strain after playing for the reserves this week.
Source: PA
Source: PA