In the footage, which was taken just outside of Citizens Bank Park in Philadelphia at some point during the Pinstripes' win over the Phils on Tuesday ... you can see a man in an Aaron Judge jersey in a heated argument with a guy in a black shirt moments before things turned violent.

The two appeared to be talking about a potential fight ... as the Judge supporter screamed he'd "dust" his competitor.

"How would you dust me?" the guy responded.

The men then traded more verbal barbs ... before the individual in the black shirt called the Judge fan a "f*****."

The vid then cuts out ... but as you can see in another clip from the area that was posted to social media -- the Yankees supporter eventually snapped at the man and slapped him so hard, he left him unconscious.

A witness tells us the fan who was smacked was taunting the man in the Yankees jersey and his group during the game ... which then continued outside the stadium. So while the video picked up outside the stadium, this all started long before the cameras were rolling.

The Philadelphia Police Dept. tells us they are aware of the incident and have launched an investigation into things.