Afroman Assault Victim -- That Knockout Is Gonna Cost Him ... Preps for Lawsuit

The woman Afromansucker punched during a concert this week is getting ready to strike back ... in court.

TMZ has learned the woman -- a 21-year-old named Haley -- hired an attorney ... who tells us Haley suffered multiple injuries to her neck, jaw and back after the attack.

Her lawyer Tim Holleman also thinks she may have suffered a mild concussion ... since she doesn't remember the punch. Even though our video shows Haley wandering on stage -- Holleman says the venue should bear some responsibility since security did nothing to remove her. Especially since Afroman has a violent history with fans.

Holleman adds ... he's not buying Afroman's claim he didn't know Haley was a woman ... because Afroman saw her when she got on stage -- something the video seems to prove.

Haley's attorney says she's already racked up medical bills, and she'll be seeking reimbursement ... if they go forward with a lawsuit.