Ryan has been sidelined since sustaining a groin injury during the 3-2 defeat at Bradford City at the end of November. However, he has responded well to treatment and has made a quicker recovery than first thought.
Speaking to the Lions' official website, assistant manager Ray Wilkins commented: "Robbie is back and could be in contention for Saturday.
Joe Dolan is also back training full-time, and we have pencilled in a reserve game on 21st January at Crystal Palace.
"He still needs a couple of weeks training before possibly playing, and because he has been out for a long while we need to treat him with kid gloves a little bit.
Mark McCammon will join in training next Monday, and the sooner we get sooner we can get the big guys back the greater our chance of progressing further up the League.
"The main thing is that we are getting a few players back now, which is good news for us all."