Czech Michalek, the franchise's third-leading goal scorer in the regular season, initially injured his left knee in a March 18 game against the Atlanta Thrashers.
Although he returned for Ottawa's last two games of the regular season, Michalek limped off the ice in the second period of Wednesday's 5-4 win over the Pittsburgh Penguins in the opening game of their first round Eastern Conference series.
"We knew he had issues and a problem," Senators general manager Bryan Murray was quoted as saying on the team's official website (http://senators.nhl.com/).
"We asked him if he wanted to continue to try to play. We talked to the doctors, we talked to his agent, we talked to Milan.
"He definitely wanted to give it a go and, unfortunately, he lasted only a very short time in a game of importance."
(Writing by Mark Lamport-Stokes in Los Angeles; Editing by Ken Ferris)