The victim was in Bentleys Bar when he was approached by an unknown man and punched.
The man was knocked out and fell to the floor. He suffered facial injuries.
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Police have now released CCTV of a man they are tracing in connection with the attack on Sunday, October 27, at 4.30am.
In a statement, South Yorkshire Police said: “We are now looking to trace the man pictured as we believe he could help us to progress the investigation.
“Do you know him? If you can help, or saw what happened that night, please report information using 101, investigation number 14/162631/19.
“You can also speak to Crimestoppers anonymously on 0800 555 111.”