Donald Trump just sent a message to Roger Goodell -- saying kneeling during the national anthem should no longer be an option ... it needs to be a league rule.
Trump continued his Twitter attack against kneelers Tuesday morning -- first, saying the booing at the Monday Night Football game was the "loudest I have ever heard" ... due to the Dallas Cowboys protest.
"Great anger," Trump added.
Then ... a message aimed at Roger Goodell.
"The NFL has all sorts of rules and regulations. The only way out for them is to set a rule that you can't kneel during our National Anthem."
We're reaching out to the NFL for comment -- but the multiple statements from NFL team owners over the weekend suggest they would not support that rule.
The NFL has all sorts of rules and regulations. The only way out for them is to set a rule that you can't kneel during our National Anthem!