Such a player would need to shatter ALL records of performance for the given position, while being a part of a team that sets the standard for wins, in particular, in the playoffs and winning SuperBowls, and doing so over a reasonably long enough career (probably ten years would be the minimum). At the same time, said player would need to win multiple MVP awards and Outstanding Player awards, as well as being widely considered a good guy and gracious winner.
As an example, let's talk runningback. He would need to average 6+ yards per carry for his career, shatter all individual season and in-game total rushing yards and rushing TD standards, be able to get the tough yards as well as being the greatest breakaway back of all-time, be a very good pass blocking RB when in passing downs, as well as ALSO setting the standard for receptions by a runningback.
So in this example, over a ten year career, he'd have to rush for 20,000 yards, 200 rushing TDs, while chipping in with a position best 850 career receptions for 8,000 receiving yards and an additional 50 receiving TDs. As well, he would have never missed a game, would win 7 League MVP awards and 6 SB MVP awards on a team with a perfect 6-0 SB record.
If the above were all accomplished, he could, without argument be called the GOAT until such time that somebody else came along to obliterate those accomplishments.