Reimer was attempting to recover a rebound when backchecking Leivo seemed to make contact with the goalie's head. Reimer lay on the ice before skating with assistance to the locker room.
Reimer, making his second start in a row (fourth appearance), was replaced by Jonathan Bernier.
With the Maple Leafs' American Hockey League affiliate, the Toronto Marlies, traveling to Rochester, N.Y., Brett Willows of the University of Toronto was the emergency backup.
Willows reportedly arrived right before the start of the third period.
After the 3-2 loss, Maple Leafs coach Randy Carlyle said Reimer had a headache but the expectation was he would be able to practice Friday.