Officers believe the man could hold vital information about an attack on a 37-year-old man outside Dempseys on Hereford Street in the city centre at around 6.35am on June 3.
The victim was punched in his face, causing him to fall and hit his head on the pavement.
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South Yorkshire Police said: “Extensive enquiries have been undertaken to trace those involved, but police are now asking for your help.”