The on-loan Liverpool goalkeeper signed in September, but is yet to see any top-flight competitive action.
That could change, however, after Union Berlin's defeat to derby rivals Hertha Berlin, with No1 Andrea Luthe coming in for heavy criticism.
Asked about Karius being called up, Fischer said: "I will look at the whole thing again so that I can express myself accordingly.
"Because I think that the impressions from one game can sometimes be misleading."
If Karius plays, he will see action again in the Bundesliga four and a half years after his last game for Mainz 05. But if Fischer sticks with Luthe, it's expected Karius will seek to cut short his loan in January.