The 24-year-old was walking along the towpath at midday in the direction of Rochdale when he was approached by two men at the junction of Bacup Road and Pexwood Road.
They demanded that he handed over some money. He refused, was punched and a melee ensued.
One of the men produced what was described as a Stanley knife and cut the victim's left forearm, inflicting a three inch wound. His right thumb was also cut.
The two men fled empty handed along the towpath in the direction of Todmorden.
One of the suspects is described as a white male, aged 25-30, scruffy and unshaven, with short dark brown hair and wearing a dark coloured hooded top.
The other suspect is described as a white male, aged 25-30, about 5ft 10in tall, of slim build, with short, mousey light brown hair and wearing a blue jumper.
Det Insp Tony Nicholson, of Calderdale CID, said there was some suggestion that the two suspects were under the influence of alcohol.
Anybody with information about the incident is asked to contact Calderdale CID on 01422 337085 or Crimestoppers on 0800 555111.
Police have arrested a 31-year-old man and a 33-year-old man in connection with the offence and have released both witrhout charge.