"His return would be my perfect Christmas present but the weather is causing problems," Millen told BBC Radio Bristol.
"When you've had a bad knee you don't want slippery grass, so it's held him back. It'll be some time in January."
The £2.25m signing had hoped that he would be able to return to action before a vital run of matches over the Christmas period.
Millen's side, which lies five points clear of the Championship relegation zone, face four games in nine days beginning with Reading on Boxing Day, followed by Crystal Palace, Cardiff and QPR.
Source: BBC Sport
Source: BBC Sport