Cleveland Browns player Myles Garrett is facing a lengthy ban for a "totally inexcusable" attack on Pittsburgh Steelers quarterback Mason Rudolph.
With eight seconds left Garrett, 23, wrestled Rudolph to the ground after he had released the ball, ripped off his helmet and swung it at his head.
A brawl erupted and defensive end Garrett was ejected from the game.
Pittsburgh's Maurkice Pouncey, who was also ejected, said Garrett "could have killed" Rudolph.
"What if he'd hit in him the temple?," said the Steelers centre.
"That is embarrassing, what I did was foolish," Garrett said. "I made a mistake, I lost my cool and that's on me. It's going to come back to hurt our team."
Browns quarterback Baker Mayfield called his team-mate's behaviour "dangerous" and "totally inexcusable" while Rudolph called Garrett a "coward" and accused him of using the helmet as a "weapon".
"That's just endangering the other team. He's going to get suspended. We don't know how long and that hurts our team," added Mayfield.
Fox commentator Joe Buck, while covering the game, described the incident as "one of the worst things I've ever seen on a professional sports field".
The Browns beat Pittsburgh 21-7 for only their fourth win of the season - ending the Steelers' four-game winning streak.
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The incident started when Garrett tackled Rudolph to the ground well after the quarterback had thrown the ball to running back Trey Edmunds.
Rudolph grabbed at Garrett's helmet. Garrett then did likewise and managed to rip off Rudolph's helmet before using it to strike the quarterback in the head.
Pittsburgh players were incensed, with Pouncey throwing a couple of punches at Garrett, before trying to kick him in the head after he was wrestled to the ground.
Pouncey and Larry Ogunjobi of the Browns will join Garrett in facing further NFL punishment after also being sent from the field.
"I've never seen that in my life," Cleveland coach Freddie Kitchens said. "I'm embarrassed, Myles is embarrassed. He understands what he did. He understands what he did is totally unacceptable."
Browns safety Damarious Randall had already been ejected in the third quarter for a helmet-to-helmet hit on wide receiver Diontae Johnson.
Pouncey told reporters afterward that Garrett should "absolutely" be suspended for the rest of the season.
"We'll see how serious the NFL is about their players," said Pouncey, who also said he didn't even remember what he did after Rudolph was hit.
Asked whether he faces a possible suspension, too, Pouncey replied, "At this point, who cares? My man got hit in the head with a helmet. I'll accept whatever penalty it is. At that point, it's bigger than football. It's protection."
The longest suspension for an on-field incident in NFL history is the five games that Albert Hanyesworth received in 2006 for stamping on another player's head.
Aside from the Garrett incident, Rudolph, 24, had a poor game as he completed just 23 of 44 passes for 221 yards with a touchdown and four interceptions as his team slipped to a 5-5 record.
The Steelers and Browns meet again on 1 December.
"A win's a win," Garrett added. "I don't think it's overshadowed by what happened in eight seconds."
The 23-year-old, the number one pick in the 2017 draft, has already been punished by the NFL this season.
He was fined $10,527 (£9,000) for an excessive facemask penalty in week one against the Tennessee Titans and then was fined $42,112 a week later for a roughing-the-passer penalty that injured New York Jets quarterback Trevor Siemian, ending his season.
Earlier this season, the Browns released safety Jermaine Whitehead after he sent threatening messages on social media to people who criticised him, which contained death threats and racial slurs.