The Nottingham Forest striker and his girlfriend were confronted at a party by a woman who claimed she had sex with him.
When another reveller tried to calm the situation down, the footballer floored him with two punches, fracturing his cheek bone and cutting his eye socket.
Mr Henderson, 32, pleaded not guilty to a charge of inflicting grievous bodily harm to John Bell on January 1, 2012.
'Identified his boxer shorts'
At St Albans Crown Court, prosecutor Alan Richards said Mr Henderson and his girlfriend Lucy Caplan had a meal at Darcy's restaurant in St Albans before going into the bar where they were confronted by 48-year-old Hazel Kent, who was not known to anyone involved in the case.
Mr Richard said: "For reasons best known to herself, Miss Kent behaved somewhat unpleasantly towards Mr Henderson and Miss Caplan, asserting on more than one occasion, in Miss Caplan's presence, that she had recently had sex with Mr Henderson. There is no reason to believe that this was anything other than pure fiction."
Ms Kent claimed that she could describe Mr Henderson's home in Markyate and correctly identified the brand of his boxer shorts. Mr Richards said: "Understandably this upset Miss Caplan and perhaps caused some tension between her and Mr Henderson. Equally understandably, Mr Henderson was keen that Miss Kent should leave him and Miss Caplan alone."
The footballer was at the bar at around 1.30am in the morning when Mr Bell, who he did not know, started to talk to him. Barman Jack Oakley said he believed Mr Bell was trying to calm the situation down.
Mr Richards said: "It is possible that his intervention caused Mr Henderson, who was already, the crown say, upset and angered by Miss Kent's behaviour, to perceive Mr Bell as somehow associated with her.
"What is absolutely clear, say the prosecution, is that Mr Bell posed no threat to Mr Henderson whatsoever. He was significantly smaller and less athletic. He was unsteady on his feet. He was in no position to challenge Mr Henderson physically and had no reason to do so.
'Knocked to the floor'
"Even if the intervention was unwelcome, it could not even begin to justify what Mr Henderson then did. He punched Mr Bell in the head twice. It may be that the first blow did not actually connect with Mr Bell, but the second certainly did and Mr Bell was knocked to the floor."
Mr Bell suffered a fractured cheek that required surgery to free a trapped nerve and treatment for a cut. Police were called and Henderson was arrested outside the restaurant.
In a police interview Mr Henderson said he acted in self-defence, believing Mr Bell was going to attack him.
Today the jury was played 11 minutes of footage from the bar area, which showed Ms Kent waving a plastic champagne flute at Mr Henderson, before Mr Bell approached the footballer at the bar. Henderson is seen to throw two punches with his right fist.
Mr Bell said he had no memory of how he ended up in Watford General Hospital. He said: "I was happy. I was with friends. It was New Year's Eve."
The case continues.