The NFL was quick to punish Kareem Jackson for his hit on Musgrave ... suspending him four games for "violations of unnecessary roughness rules" against the Packers.

In a letter to Jackson, the NFL's VP of football operations Jon Runyan explained the decision ... saying, "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."

"You could have made contact with your opponent within the rules, yet you chose not to."

Runyan also pointed out it wasn't Jackson's first offense ... as he was hit with several personal fouls for violating player safety-related rules.

Jackson can appeal ... but as it stands right now, he is slated to miss the Broncos' matchup against Kansas City, Buffalo, Minnesota and Cleveland.

Broncos defensive back Kareem Jackson received a racist message following his ejection from the Packers game Sunday ... and he was so angry with the comment, he called out the sender publicly.

The DM was sent to Jackson's Instagram account just after he was booted from Denver's 19-17 win over Green Bay for illegally hitting Luke Musgrave in the head and neck area.

The message, apparently sent from a man named Philipp, read, "Stop behaving like a f***ing n** 🤡"

Minutes later, the 35-year-old safety -- who was actually ejected from the Broncos vs. Commanders game earlier this season as well -- reposted the message ... and was clearly fuming over its content.

"I wanna make sure you get the attention you're looking for," Jackson said. "I want your friends and family to know what type of racist person you really are over a game."

"I'll repost all this week so it doesn't go unnoticed. Don't worry, IG tough guy."

Jackson later revealed he got another inappropriate message on the app ... one he was also furious about.

"Another IG tough guy," Jackson wrote after reposting the DM to his 120,000 followers. "This what we deal wit because we are athletes on a regular. Disgruntled fans that hide behind social media knowing it wouldn't end well in person. Enjoy this fame Daniel."

This is the second time a high-profile player has called out fans for their messages on Instagram this season ... earlier this year, Vikings star Alexander Mattison said he got over 60 hate notes from fans, with some including the N-word.