The assault took place on the footpath from Haddington Road to Northfield in Tranent, Edinburgh, at around 11am yesterday.
The 26-year-old victim was knocked to the ground by two men who encouraged their dog to bite him on the head.
The only description police have of the attackers is that they are in their 20s and had a brown bull mastiff dog.
They may have also been seen in a white Vauxhall Corsa with a black roof, police said.
Officers are appealing for any witnesses to contact them.
Detective Constable Andrew Hope said: "The victim was attacked with such severity by both men, who also encouraged their brown-coloured bull mastiff dog to bite him on the head, that he will need plastic surgery.
"We do not have any description of the men who carried out this assault, but believe they had been seen earlier in a white Vauxhall Corsa, which may have had a black roof.
"Anyone who saw this assault take place, or can help us identify the two men involved, is asked to contact Police Scotland on 101 or provide the information anonymously to Crimestoppers on 0800 555 111."