Fischer will miss the 20222023 Europa League game against Malmö on Thursday, Union Berlin said.
All other players and members of the coaching staff have tested negative, the German side added.
While Baumgartl is not registered to play in the Europa League, Maciejewski has not been in the squad so far in the Bundesliga season.
He was only on the bench against Braga last month.
Union Berlin are third in their Europa League Group D and are yet to secure a point in the competition.