It was serious enough for him to be taken to the hospital in an ambulance. How the players on the field reacted is the real story. And it was beautiful:

https://notthebee.com/article/this-one-play-lasted-more-than-6-minutes-in-last-nights-preseason-nfl-game-and-the-reason-why-is-special

That was pretty crazy and yet awesome to see. I thought it was also crazy how quickly the Atlanta trainers were out there and the Lions trainers took their time. Yes it was closer to the Atlanta sideline but there was plenty of time for a lions trainer to get there and there were still none. But when they did get there they already had the backboard and neck brace.