The Senators dressed Patrick Wiercioch in place of Michalek, giving them seven defensemen for the game.
Michalek had two goals and six assists in 13 games so far this season, including an assist in 18:09 of ice time Tuesday night in a 2-0 win against the Buffalo Sabres. He had been skating on Ottawa's top line with Mika Zibanejad and Daniel Alfredsson.
The team gave no further update on Michalek's condition. He did not take part in a sparsely attending morning skate Wednesday. Coach Paul MacLean said afterward that, other than Andre Benoit taking Wiercioch's place in a swap of defensemen, there were would be no lineup changes from the Buffalo game.
Here are the updated lines the team was using in the first period of the game in Pittsburgh: