Copeland, 31, and his lady friend, Katrine Saltare, 28, were arguing as they left 1OAK just before 4am when a 22-year-old man, Sharoy Blery Murdock, interrupted them, the New York Post reported.
When Copeland, who used to play for the New York Knicks, told the man to back off, the suspect allegedly pulled out a knife and plunged it into the athlete's abdomen and elbow.
He then turned on Saltare, stabbing her in the breast, buttocks and arm, sources told the Post.
Two Atlanta Hawks players -- Pero Antic and Thabo Sefolosha -- were arrested at the scene for trying to prevent police from setting up a crime scene.