The official told CNN's Coy Wire that there's evidence McCoy and three other men punched and kicked one of the officers while he was on the ground in a fight that police say began over a bottle of champagne.
The incident happened early Sunday at a nightclub in Philadelphia, according to police. It involved McCoy, another as-yet unnamed former NFL player who played with him on the Philadelphia Eagles, and two of McCoy's former college teammates, the league official said.
Neither McCoy nor any of the others have been charged yet, according to authorities. The NFL official said he expects McCoy to be charged with aggravated assault.
No charges were expected until at least Wednesday, according to McCoy's attorney, Larry Krasner.
McCoy's agent, Drew Rosenhaus, declined to comment.
The Bills released a statement Monday saying they were aware of the reports and "in the process of gathering more information."
The incident allegedly began over a bottle of champagne, which led to pushing and shoving, according to a police account.
One of the off-duty officers was punched and knocked to the ground, according to police.
While the officer was down, all four men repeatedly punched, kicked and stomped him on the body and head, according to police. The second officer intervened with security guards to break up the fight.
Police said all of the parties to the fight were ejected from the club. Their ejection was captured on surveillance video, but the fight itself was not, police said.
The first officer was treated for a broken nose, broken ribs, a sprained thumb and a cut near his eye, according to police.
The officer who helped break up the fight was admitted to a hospital for a possible skull fracture and a cut over his left eye, which took eight stitches to close, according to police.
It is not clear whether the two injured men were working security for the club or were customers.
McCoy, a star running back with the Bills, has been in the NFL since he was drafted out of the University of Pittsburgh in 2009.
He played the first six years of his career with the Philadelphia Eagles. He was traded to Buffalo in 2015, signing a five year, $40 million contract.