Mittelstadt (lower body) has been ruled out of Saturday's tilt in Toronto, per Jim McBride of the Boston Globe.
Mittelstadt was involved in a knee-on-knee collision in Thursday's 3-2 overtime win over Ottawa, but he managed to return to the game. However, he will miss at least one contest, and a timeline for his return is unclear. He has four goals and five assists across 15 games this season. Jeffrey Viel will probably replace Mittelstadt in Saturday's lineup versus Toronto.