The 35-year-old victim was attacked from behind and knocked unconscious in the incident, which happened on Harehills Lane between 8.30pm and 9pm on Wednesday, January 30.
Police say he had just visited the International Foods store on Harehills Lane and was walking towards York Road when he was assaulted.
Officers are particularly keen to speak to a man – described as young and possibly Polish – who went to his aid.
Leeds District CID’s PC Ruby Price said: “We are investigating this serious assault in which the victim appears to have been targeted at random while walking on the street.
“We would very much like to speak with any witnesses and in particular a young male who helped the victim to his feet in the aftermath of the assault.
“It is possible this man, who the victim thought was Polish, may well have seen what took place.”
Anyone with information is asked to ring Leeds CID on 101, quoting log reference 13190062382, or Crimestoppers anonymously on 0800 555111.