The 56-year-old victim had come across two men arguing in the ginnel which runs from Ashgap Lane to Gladstone Street.
He was punched by one of them and hit his head on a wall before falling to the ground.
The men then made off, leaving the victim with facial injuries which needed hospital treatment.
Wakefield District Police are now seeking witnesses or anyone with information about the assault, which took place between 5.30pm and 5.40pm that day.
PC Joanne Blackmore, of Wakefield Police, said: “We are investigating what has been a nasty assault, which was unprovoked.
“The victim needed hospital treatment as a result of this attack and I would like to speak to anyone who may know the identity of the suspects or who has any information at all."
Both of the men were described as white, 6ft and of Eastern European origin.
One of them was of medium build, aged 25 to 30 and had cropped, brown hair. He was wearing a black hooded top with a motif on the front, jeans and trainers.
The other was stocky with cropped brown hair, aged 25 to 30 and wearing a black leather jacket, jeans and shoes.
Anyone with information is urged to call PC Blackmore on 101, quoting crime number 13180001792.
Information can also be given anonymously to the independent Crimestoppers charity on 0800 555 111.