Romero and a fan, 25-year-old Robert Eaton, were involved in an incident immediately following Thursday night's game between the Phillies and Rays at Tropicana Field.
St. Petersburg police spokesman George Kajtsa said the fan asked players leaving the field for autographs.
"The player ignored him and kept on going," Kajtsa said of Romero. "So the victim starts yelling expletives and also making references to some things I can't mention because it's under investigation. That apparently angered the player, who then returned. The two had a shouting match and at one point the player hit him with an open hand -- or he slapped him on his neck."
Eaton told the St. Petersburg Times that he said something to Romero like, "How about you get me some juice?" That is in reference to Romero's 50-game suspension this season for violating the league's policy against performance enhancing substances.
Police never had the opportunity to interview Romero, who left Florida for a weekend series in Toronto.
"Remember, this is a simple battery," Kajtsa said. "No weapon was used. There were no injuries to the victim whatsoever, except as he says in the report he was embarrassed because it happened in front of other fans and his family.
"This is a very minor incident so the state's attorney will probably concentrate on more serious matters, but it will be up to the state's attorney to make the determination."
Todd Zolecki is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.