Keefe said Wednesday he was told the injury would take "at least seven days" to heal from the time it occurred. Andersen has not practised with the Leafs at all this week since re-aggravating an existing lower-body injury.
The 31-year-old Andersen last played in Friday's 4-3 loss to the Calgary Flames in which he stopped 14 of 18 shots. Jack Campbell started Saturday's game against the Flames, a 2-0 win, and Andersen was not on the Leafs' bench as Campbell’s backup.
Andersen is 13-8-2 this season in 23 appearances for Toronto, with a 2.91 goals-against average and a .897 save percentage.
Campbell will get the start between the pipes for the Maple Leafs in Thursday's game at the Ottawa Senators. The 29-year-old is 4-0-0 this season and hasn't allowed a goal in each of his last two starts.