Jahlil Okafor Fight -- 2nd Victim Threatens Lawsuit ... 'I'm Hurt & Embarrassed'

The man who was KO'd in the 2nd Jahlil Okafor street fight claims it was the NBA star who knocked him out ... and now he's threatening to fight back in a HUGE lawsuit.

The man behind the allegations does not want to reveal his I.D. -- but his attorney, Michelle Newton, tells TMZ Sports he's a 27-year-old executive assistant who lives in Boston.

We know the man was knocked out cold during the melee at Storyville nightclub last week -- but it was unclear from the footage if Okafor was his assailant.

But the victim's attorney tells TMZ Sports they're POSITIVE it was Okafor -- and he needed 10 stitches to close the gash on his eye after Jahlil knocked him out.

Newton says a lawsuit is now on table -- though they have NOT filed any paperwork yet.

"We're always interested in resolving any matter prior to ligation," Newton tells us .... "Should we be unable to reach an agreement [with Jahlil], civil litigation is the only available option."

Translation -- settle or we'll see you in court.

There's more ... Newton says her client has experienced pain and embarrassment as a result of the fight.