E-fits of the man and woman have been released by police investigating the incident on Town Fields last month.
Just after midnight on September 15, the 50-year-old man was walking across Town Fields when he was approached by a man and a woman who began to engage him in conversation.
The pair are said to have asked the man for money and when he refused, reportedly became aggressive.
PC Chris Beaumont, the investigating officer, said: “The man who threatened the victim was reportedly carrying a knife, although thankfully, no one was injured during the incident.
“The man and the woman then took the victim’s mobile phone, before leaving towards Lawn Avenue and Thorne Road.
“I’d like to ask for your help to identify the two people in the e-fit images. If you recognise them, think you know who they might be, or have any information about the incident, please get in touch with us.”
Anyone who can help should call 101 or Crimestoppers anonymously on 0800 555111 quoting incident number 6 of 15 September 2017.