The England international suffered a broken foot during a match away at Everton three months ago.
However, despite his presence during recent sessions, head coach Steve Clarke has insisted that he will not rush the 30-year-old back into action.
"There have been absolutely no complications. It's just been a little bit slower than we thought because medicine and recovery from injuries is not an exact science," the Express & Star quotes Clarke as saying. "On the 12-week mark he was getting close and since then his progress has been good. He's not too far away but I won't tell you he'll be ready in a week, two weeks, three weeks or four weeks.
"I'll just wait and let nature take its course. Ben's been back working outside for a couple of weeks now. He hasn't joined the group for full training but he is back working with the goalkeeping group, which is just another step in the right direction for him."
Boaz Myhill has deputised in Foster's absence.