The 29-year-old last played in Saturday's 5-3 loss to the Vancouver Canucks, and was planning to skate in Monday's contest, an eventual 3-2 shootout loss to the Los Angeles Kings, but reported feeling nauseated prior to the game -- according to a story from NHL.com.
Leafs head coach Ron Wilson expressed shock that the injury occurred, since Armstrong gave no indication that anything had happened, according to the piece.
Armstrong had already been out of the lineup from mid-October until early December after sustaining a sprained left ankle. He has recorded just one assist in nine games.
To fill the roster spot, the Maple Leafs recalled forward Darryl Boyce from the Toronto Marlies of the AHL.