Nikci was rushed to hospital yesterday after what started off as a cold and fever turned out to be meningitis.
He will spend the next seven days under observation after pulling out of the Hannover squad to face Hamburg over the weekend.
"We wish Adrian a speedy recovery," said the club's director of sport Jorg Schmadtke. "He is in good hands with the doctors and can hopefully leave the hospital again soon."
Andreasen's recovery is guaranteed not to be so quick, after he went under the knife in Augsburg today after tearing cruciate knee ligaments last week.
The operation was a success, according to the Bundesliga club's website, but the Dane nevertheless faces a spell of six months on the sidelines.