The 24-year-old is facing a charge of taunting a police K-9 after allegedly lifting his shirt, barking and pounding his chest.
The incident is said to have taken place before the Raiders suffered a 38-35 defeat to Pittsburgh Steelers on Sunday.
Police deputy Maria Watts, who was the handler of Bandit the dog when the alleged taunting occurred, told WTAE Pittsburgh television that Armstrong was shouting "hey dog, hey dog" in the dressing room before the game.
"Bandit was very agitated," she said. "He wanted me to let him go. I imagine with his training and experience he would have gone to his target who was taunting him. I don't want to speculate on what he would have done. No-one in the department, from the sheriff down, is happy about this."
The charge is a third degree felony in Pennsylvania.