TMZ:
NBA legend Paul Pierce is being sued over an altercation with a security guard in the stands at the Conor McGregor vs. Khabib Nurmagomedov fight ... TMZ Sports has learned.
Mostello claims he was told Pierce grabbed the woman by the arms and pushed her to the side -- so he stepped in front of Paul to try and stop him.
But, Mostello claims Paul didn't stop -- instead Paul screamed "F*ck you" and accused Mostello of being a racist.
In the suit, filed by attorney Jesse Sbaih, Mostello accuses Pierce of defamation and intentional infliction of emotional distress.