Nicki was admitted to hospital after complaining of a fever and forced to pull out of Hannover's match against Hamburg. He will spend the next seven days under observation.
The club's sporting director Jorg Schmadtke said: "We wish Adrian a speedy recovery.
"He is in good hands with the doctors and can hopefully leave the hospital again soon."
Schmadtke also revealed that midfielder Leon Andreasen has undergone an operation on his knee and will be on the sidelines for at least six months.