The incident happened at around 9pm on Saturday in King Street, Port Glasgow.
The victim was taken by ambulance to Inverclyde Royal Hospital. He has since been released.
Detective Constable Euan Keil of Greenock CID, said today: “The man had been drinking in the bar and had popped outside for a cigarette when he was attacked by a man who had also been in the pub.
“Officers are carrying out inquiries into this incident and are viewing CCTV footage in an effort to gather information on the person responsible.
“There were a few people in the area at the time of the assault and I would ask anyone who saw what happened or has any information that will assist us with our enquiries to contact police.
“I am particularly keen to speak to a man who came to the aid of the injured man following the attack.”