Trump's belief those players should be "fired" prompted further protests when the Star-Spangled Banner was played ahead of NFL matches over the past two weekends, ranging from kneeling, raising a fist and, in a few instances, not even appearing on the sideline during the anthem.

While fewer players took stances on Sunday, Trump's incendiary comments, which included the phrase ''son of b****'' to describe those who do not stand, were provocative to many.

And Ahmad Nassar, president of the NFL Players Association's marketing arm, believes those who protest realise they must do more than take a pre-game stance if they want to see social and political change enacted.

"I think players are interested in as broad a sense as possible," Nassar explaines.

"Malcolm Jenkins of the Philadelphia Eagles has written an op-ed in the Washington Post about what else he and a players' coalition are working on beyond the field - things like (increasing) voter registration, lobbying congress, going on police ride-alongs, and I think that's really the next phase of this story.

"It's not so much what is happening on the field prior to the game but what is happening off the field. These athletes care very much and respect their communities, fans, the military - all these things that you've seen brought up over the last 10 days. That's hopefully where you will see the real positive change start to take root.

"The players right now are in the early stages of trying to figure out where this goes from here.

"It's one of the things they're looking at - what are the best ways to get that message out? It's clear that this can be player-led but it can't be only player-driven, it has to have buy-in from the fans and the community but I think there's an appetite there.

"What it can't be is limited to what happens before the game starts and I think Malcolm especially, but all the athletes know that, and they've always known that."

The concept of taking a knee during the anthem first originated last pre-season when then-San Francisco quarterback Colin Kaepernick did so in response to what he considered racial oppression.

On Sunday around half of the 49ers' roster knelt for the anthem, way more than any other team.

Kaepernick remains unemployed despite being a starter last year amid the theory he has been shunned for his political views.

Yet in weighing into the argument Trump has forced NFL players, coaches and owners alike to tackle the response to issues Kaepernick wanted to highlight.

Nassar added: "We hold out hope it can be a net positive in that it's brought players and teams, if not together, at least it's forced some hard conversations that were potentially being buried or avoided to this point. I view that as a good thing."

The NFLPA may also have a crucial role in whether London one day gets its own franchise.

The Collective Bargaining Agreement between the league's 32 owners and its players is due for renewal in 2021, when the feasibility of an international team is bound to be on the agenda.

Nassar knows players will want many issues ironed out if a franchise is to one day call England home, though he does not believe they are insurmountable.

"If your employers were to come in and say, 'We're opening an office in New York from London, no questions asked', you would have a lot of questions," he said.

"How do my benefits work? How do the logistics work? How do the labour laws work? How do taxes work?

"Those are all issues that I think can be worked out, but they need to be figured out. I don't think that they have been yet."