MacLean said after the team's morning skate for their game Tuesday against the New York Islanders that Michalek underwent arthroscopic surgery on his right knee to clean up debris.
The Czech forward is out indefinitely.
The 28-year-old missed a month of action and then tried to play through the pain earlier in March, recording an assist in both games he played. All told, he's registered two goals and eight assists in 15 games for Ottawa in 2012-13.
Acquired in the 2009-10 multi-player trade with San Jose that saw Dany Heatley head to the Sharks, Michalek led the Senators with 35 goals last season.
The Senators head into the Islanders contest in fifth place in the Eastern Conference despite long-term injuries to top players in all three positions — forward Jason Spezza, defenceman Erik Karlsson, and goalie Craig Anderson.