While injuries are common in the sport, multiple observers said it was the first time they had seen an ambulance enter the field to treat a player.
"I've never seen anything like that before," said former quarterback Kurt Warner on NFL Radio.
The NFL Players Association said they had been in contact with players from both teams.
"The NFLPA and everyone in our community is praying for Damar Hamlin. We have been in touch with Bills and Bengals players, and with the NFL. The only thing that matters at this moment is Damar's health and well being," the union said on twitter.
"Please pray for our brother," tweeted Bills quarterback Josh Allen while players from across the league took to social media with similar messages.
Some fans traveled to the University of Cincinnati hospital where Hamlin was being treated, as did teammate Stefon Diggs.
The incident, broadcast during the weekly nationally televised Monday night game, drew reaction from athletes in other sports.
"The safety of players in all sports is always the most important," said NBA star LeBron James.
"It was a terrible thing to see and I wish nothing but the best for that kid. I am a huge fan of the NFL and football and you never want to see anything like that happen," he added.
The NFL did give any indication of when the game might be continued or replayed.
Both the Bills and Bengals have already secured places in the NFL's playoffs, but the game was important for both teams as they sought to get the number-one seed in the AFC.
That would bring a bye and home field advantage throughout the post-season. The NFL regular season is scheduled to end on Sunday with the playoffs starting the following weekend.