Officers responded to Jamaica, Queens, at about 10 a.m. Tuesday after receiving 911 calls from inside the restaurant.
The New York Post reports that the victim was stabbed in the vicinity after getting into an argument with a group of men.
Witnesses tell the Post that the man was chatting on his cellphone and appeared composed despite the knife in his back.
There have been no arrests.
The man was treated at Jamaica Hospital Medical Center.
Photos show the man wearing a white T-shirt that is stained in blood.