Conor McGregor is firing back at the security guard who claims the UFC star injured him during that crazy 2016 bottle-throwing incident at the Nate Diaz fight presser ... saying his accuser sucks at his job.
The man who filed the lawsuit is William Pegg -- who was working for MGM security on Aug. 17, 2016 ... when Nate and Conor threw bottles and cans at each other during a heated exchange before UFC 202.
Pegg says McGregor chucked a can that struck him in the left shoulder and injured him.
But now, McGregor is telling his side of the story ... saying Pegg failed in his duty to provide security at the event and "did nothing" while Nate and his crew initiated the incident by throwing bottles at Conor first.
"Had [Pegg] met his obligations not only as a security guard on duty during the press conference, but moreover, the person purportedly in charge of security, and protected [Conor] and his friends and family, than this entire incident would have been avoided."
Conor also admits to throwing objects that day (obviously, it was captured on video) ... but says there's NO PROOF the projectile that struck Pegg was thrown by him.
The case is due back in court later this year.