Jeff Savidge, who witnessed the fall, said play on the field had stopped for a television timeout when the a man suddenly landed across the neck and shoulders of a spectator.
"He kind of bounced off him and landed about two rows down on the ground," Savidge said. "I heard somebody scream from behind us."
The man who fell got up and said he was OK but was stopped by security from leaving, Savidge said. Arriving emergency workers put a neck brace on him and carried him away on a chair stretcher, he said.
A security officer, meanwhile, held the head of the fan who was struck to keep him from moving as he sat upright in his seat awaiting medical attention, Savidge said. He was carried out on a backboard.
"Yesterday's reckless and dangerous incident at Ralph Wilson Stadium is an example of the type of behavior that gives Buffalo a bad reputation and that can never be tolerated, dismissed or accepted," Erie County executive Mark Poloncarz said.
The Bills lease the stadium from the county.
Brandon said the team is cooperating with an Erie County Sheriff's investigation of the incident. Sheriff's spokeswoman Mary Murray, citing the investigation, did not release the names of the men. She said charges are possible.
In September 2012, a 20-year-old fan from Tennessee died when he tumbled over a railing at the Georgia Dome and struck a man below during a college game.
Since 2003, more than two dozen cases of fans falling at stadiums have been reported across the United States, according to the Institute for the Study of Sports Incidents. The institute is part of the National Center for Spectator Sports Safety and Security.