This was not the news the Tampa Bay Lightning needed to hear.

Forward Ryan Callahan underwent an emergency appendectomy Monday night and will be out indefinitely.

Callahan reportedly started feeling abdominal pain Monday afternoon and was taken to Tampa General Hospital for surgery.

There is no timetable for his return, a team source told ESPN.com's Pierre LeBrun, but he will almost certainly miss at least Game 6 between the Lightning and Montreal Canadiens on Tuesday.

Lightning head coach Jon Cooper updated Callahan's status Tuesday morning.

Jon Cooper calls Ryan Callahan "extremely doubtful" to play in Game 6 tonight ... Less than 24 hours after having an emergency appendectomy. — Stephen Whyno (@SWhyno) May 12, 2015

Cooper added that "crazier things have happened," leaving the door open ever so slightly for a Game 6 appearance, and believes Callahan's timetable should be set at days rather than weeks.

Callahan has three assists in 12 games this postseason.