McAmmond had lost possession of the puck early and skated behind the Philadelphia net when he was checked hard by Downie at 2:39 of the second period. Downie's feet left the ice before the violent collision.
Senators team doctor Don Chow immediately went to McAmmond's aid even as players from both teams squared off in the aftermath.
Ottawa enforcer Brian McGrattan went after Downie, who was assessed a match penalty for the hit on McAmmond. McGrattan got a misconduct and a game misconduct in addition to his major for fighting with Philadelphia's Jesse Boulerice.
Senators forward Patrick Eaves stood guard while Chow attended to McAmmond before he dashed off and skated the length of the ice to summon and hurry along the ambulance technicians bringing the stretcher.
McAmmond did not move his hands or his feet while he was fully strapped onto the stretcher and taken off the ice. A team spokesman confirmed he had suffered a concussion -- and subsequently moved his extremities -- and was taken to a hospital for further evaluation.
McAmmond missed the final two games of last spring's Stanley Cup finals against Anaheim after he suffered a concussion when he was elbowed in the head by Ducks defenseman Chris Pronger and knocked out of Game 3.