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.