https://youtu.be/4HR6fhif9CUp (paste into browser for video)

The specific rule pertaining to the national anthem is found on pages A62-63 of the NFL League Rulebook. It states:
“The National Anthem must be played prior to every NFL game, and all players must be on the sideline for the National Anthem. “During the National Anthem, players on the field and bench area should stand at attention, face the flag, hold helmets in their left hand, and refrain from talking. The home team should ensure that the American flag is in good condition…
…It should be pointed out to players and coaches that we continue to be judged by the public in this area of respect for the flag and our country. Failure to be on the field by the start of the National Anthem may result in discipline, such as fines, suspensions, and/or the forfeiture of draft choice(s) for violations of the above, including first offenses.”

AND THERE IS MORE, EVEN A FEDERAL STATUTE BREACHED BY STEELERS COACH MIKE TOMLIN:

From Title 36—PATRIOTIC AND NATIONAL OBSERVANCES, CEREMONIES, AND ORGANIZATIONS

§301. National anthem

(a) Designation.—The composition consisting of the words and music known as the Star-Spangled Banner is the national anthem.

(b) Conduct During Playing.—During a rendition of the national anthem—

(1) when the flag is displayed—

(A) individuals in uniform should give the military salute at the first note of the anthem and maintain that position until the last note;

(B) members of the Armed Forces and veterans who are present but not in uniform may render the military salute in the manner provided for individuals in uniform; and

(C) all other persons present should face the flag and stand at attention with their right hand over the heart, and men not in uniform, if applicable, should remove their headdress with their right hand and hold it at the left shoulder, the hand being over the heart; and

(2) when the flag is not displayed, all present should face toward the music and act in the same manner they would if the flag were displayed.

NFL coach to Army vet player: Do NOT stand for anthem

SEPTEMBER 25, 2017

After hundreds of NFL players decided to publicly disrespect the American flag on Sunday, one player went out of his way to take a patriotic stand — and it left his coach fuming.

NFL head coach Mike Tomlin and the Pittsburgh Steelers decided to wait inside the locker room during “The Star-Spangled Banner” — but that didn’t sit well with offensive tackle Alejandro Villanueva, who went to the field alone and saluted the flag.

The patriotic gesture by Villanueva, a former Army Ranger that served three tours in Afghanistan, left Tomlin steaming.

Addressing the media after his team’s overtime loss to the Chicago Bears, Tomlin took a swipe at the Bronze Star recipient.

“I was looking for 100 percent participation — we were gonna be respectful of our football team,” Tomlin said.

Tomlin had said that before the kickoff Sunday, he held a team meeting and discussed with his players not come out of the locker room for the playing of the national anthem.

Tomlin said he intended to have the team focus on the game, and implied that by saluting the flag Villanueva was showing disloyalty to his teammates.

“Many of them felt like something needed to be done. I asked those guys to discuss it and whatever they discussed that we have 100 percent participation or we do nothing. They discussed it for an appropriate length of time and they couldn’t come to an understanding, so they chose to remove themselves from it,” Tomlin told the media. “They were not going to be disrespectful in the anthem so they chose not to participate, but at the same time many of them were not going to accept the words of the president.

Villanueva was clearly not happy with the team’s decision, and decided to make his own protest — by coming out alone and placing his hand over his heart.

This isn’t the first time Villanueva has taken a stand for the flag, either. The offensive tackle had previously slammed the protests inspired by former NFL quarterback Colin Kaepernick as something that “sends the wrong message”

In 2016, Villanueva said, “I don’t know if the most effective way is to sit down during the national anthem with a country that’s providing you freedom, providing you $16 million a year… when there are black minorities that are dying in Iraq and Afghanistan for less than $20,000 a year.”

“I will be the first one to hold hands with Colin Kaepernick and do something about the way minorities are being treated in the United States, the injustice that is happening with police brutality, the justice system, inequalities in pay,” Villanueva told ESPN. “You can’t do it by looking away from the people that are trying to protect our freedom and our country.”

FYI