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The suspect is alleged to have punched the victim in the face, causing him to fall to the ground.
The 24-year-old was taken to hospital, where he required hospital treatment for serious facial injuries.
Investigating officers have now issued CCTV images of a man they believe could be able to help with enquiries.
Detective Constable Joanne Mellor said: “This was an unprovoked and nasty assault that has resulted in a young man requiring surgery.
“We have spent the last month trying to speak to as many witnesses as possible, as well as trawling through CCTV in the area in a bid to identify the suspect.
“Now we need your help – if you saw anything that you think might help us, please get in touch.”
If you recognise the man pictured or hold information about the assault, call police on 101 quoting incident number 304 of 24 May 2015.
Alternatively, call Crimestoppers anonymously on 0800 555 111.