Jackson is appealing his suspension, a source told ESPN's Jeremy Fowler. Jackson would forfeit $558,889 in salary if the four-game suspension stands after appeal.
Jackson was ejected from Sunday's game against the Green Bay Packers for his sideline hit on Packers tight end Luke Musgrave in the fourth quarter. It was the second time this season that Jackson was ejected for an illegal hit.
"On the play in question, you delivered a forceful blow to the head/neck area of a defenseless receiver, when you had the time and space to avoid such contact," vice president of football operations Jon Runyan wrote in a letter to Jackson. "You could have made contact with your opponent within the rules, yet you chose not to."
Runyan noted that Jackson has had multiple instances of violations of player-safety rules.
Entering Sunday's game, Jackson had been fined four times this season for unnecessary roughness -- for a total of $89,670.
If Jackson's four-game suspension is upheld, he will miss games against the Kansas City Chiefs, Buffalo Bills, Minnesota Vikings and Cleveland Browns. He would be eligible to return to Denver's active roster Monday, Nov. 27.
On Sunday, Musgrave caught an 18-yard pass from Jordan Love and, as linebacker Alex Singleton closed in, Jackson arrived and knocked Musgrave to the turf. Two flags were immediately thrown. After a brief discussion among the officials, referee Alex Kemp announced that Jackson had been ejected in addition to the 15-yard penalty.
Some Broncos players said they believed Jackson had led with his shoulder and hit Musgrave slightly below the head and neck area. Green Bay players on the sideline near the hit shouted at Jackson after the play as Packers coaches quickly got between Jackson and the group.
"I know Kareem, I know his heart and I know the way he's played," Broncos coach Sean Payton said Monday before the suspension was announced. "I think it's an easy narrative to say, 'Well he's just an old-timer playing in a new-timer's game.' When you see the play, his head is removed from the forceable contact. Defenseless is a little bit more of an issue relative to the position the receiver's in.
"I just know where he's at as a player. We'll keep working on that. I know he'll keep working on it. He's someone that's smart and really wants to do the right thing. It's one of the toughest parts of our game for the officials, for the players, for all of us involved of getting that to where it's clear and easy. Sometimes, it's just not."
Jackson's suspension is the longest punishment for violating player-safety rules since 2019, when Cleveland Browns defensive end Myles Garrett was suspended indefinitely for hitting the Pittsburgh Steelers' Mason Rudolph with the quarterback's helmet. Garrett ended up missing six games and was reinstated in February 2020. Also in 2019, Raiders linebacker Vontaze Burfict was suspended for the remainder of the season (12 games) for a helmet-to-helmet hit on Colts tight end Jack Doyle. The NFL cited Burfict's repeated violations of unnecessary roughness rules for the severity of the punishment.
ESPN's Jeff Legwold contributed to this report.