South Yorkshire Police said the 21-year-old was standing outside Moo Lab on Abbeydale Road when he was called over by a man close to a black Audi.
As the victim approached the man, he was stabbed in his forehead with a knife.
The attacker fled in the direction of Sheffield city centre in a silver taxi.
The stabbing occurred at around 8.45pm on Thursday, May 5 but details have only just been released by detectives investigating the incident.
The culprit is Asian, around 5ft 10ins tall and of a stocky build. He was wearing black clothing at the time.
South Yorkshire Police said: “Officers now wish to speak to anyone who may have information about what happened.
“Were you in the area at the time? Did you see what happened? Do you know who might be involved?”
Call 101 and quote crime reference number 14/84160/22.