Cleveland Browns' Myles Garrett has been "suspended indefinitely" by the NFL after hitting Mason Rudolph with the Pittsburgh Steelers quarterback's own helmet.
Garrett is suspended for at least the remainder of the season and will not be paid during the ban.
The 23-year-old will only be reinstated with the approval of NFL officials.
Garrett said his actions were "selfish and unacceptable", apologising to Rudolph in a statement on Friday.
"Last night, I made a terrible mistake," he added.
"I know that we are all responsible for our actions and I can only prove my true character through my actions going forward.
"I want to apologise to Mason Rudolph, my team-mates, our entire organisation, our fans and to the NFL."
Pittsburgh's Maurkice Pouncey, who was also ejected after a subsequent scuffle, has been banned for three games.
Cleveland's Larry Ogunjobi has been banned for one match for pushing over an opponent in the aftermath of the incident, while both teams have been fined $250,000 (£194,000),
What happened in the game?
With eight seconds left in the game Garrett 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.
"That is embarrassing, what I did was foolish," Garrett said after the match. "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.
- NFL predictions: Texans at Ravens, Bears at Rams, Chiefs at Chargers
- Whitehead released over Twitter outburst
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.
"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."
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.