The 21-year-old Danish defender has one muscle too many in his foot.
"Mikkel still has a small problem, things are not quite right so we are looking at different way of treating the injury," Kevin Keegan told City's official site.
"He is having a scan on Thursday and, without going into too much detail, he has an extra muscle in his foot.
"It is an incredible thing in that he has a muscle in his foot which other people don't have. That is part of the problem.
"In a nutshell the treatment he will undergo has to stop this extra muscle working. It should take about three weeks and then he will return to training."
Bischoff has been a regular for the reserves this season but made one first team appearance against TNS away and appeared on the bench against Spurs.