MacLean says Tuesday the surgery was to clean up debris in the knee.
The injury caused Michalek to miss 10 games this season. He came back for two games in early March, picking up an assist in each game, but the knee kept bothering him.
Michalek has two points and eight assists in 15 games with the Senators this season. He had a breakout season in 2011-12, with 35 goals and 25 assists in 77 games.