Andrew Lewis – who had already admitted attacking 56-year-old Paul O’Donnell outside the City Ground in August 2017 – had been accused of manslaughter, but prosecutors have now dropped the charge.
Miranda Moore QC told Leicester Crown Court that doctors could not definitively say the injuries Lewis inflicted on Mr O’Donnell caused his death.
The Millwall supporter had already pleaded guilty to a separate charge of causing grievous bodily harm by punching the Forest fan during a fight which broke out before the two teams played last year.
Lewis had drunk seven pints of lager in the build-up to the match and told the court he had launched the assault on Mr O’Donnell because he had been saying “West Ham” – Millwall’s fiercest London rivals – to him.
The fracas took place before a Friday night fixture between the two clubs on the opening day of the last year’s Championship season, which Forest went on to win 1-0.
Mr O’Donnell, an avid Forest supporter, was found in a critical condition two weeks later at his home in Newark, Nottinghamshire, and was pronounced dead in hospital on 24 August, almost three weeks after he was attacked by Lewis.
He had suffered a skull fracture and bleeding on the brain thanks to the “entirely unnecessary as well as aggressive blow” from Lewis, the court heard.
Immediately after he was hit by Lewis the father-of-two was taken to hospital, but was later released. He did, however, complain of memory loss and severe headaches, Ms Moore told the court.
Forest fans at the City Ground stood for a minute’s applause in the 56th minute of a match in September last year after his death to honour Mr O’Donnell.
His partner of six years, Kimberley Dempsey, said the tribute was “lovely” but a bittersweet moment nevertheless.
“I will be thinking we shouldn’t be here doing this. He shouldn’t be dead,” she told the Newark Advertiser before the match.
“He was so funny, sharp and witty. He was also stubborn and would have an answer for everything. Once he had an idea in his head, that was it. He was a peach really.”
His sister, Margaret Barthorpe, said her brother was immensely popular in their neighbourhood and among Forest fans.
“The support we have had has been absolutely amazing. It has kept us going. He was very popular.
“We only realise how much he was appreciated now he has gone.”
Lewis, from Bromley in South London, was jailed on Wednesday for one count of causing grievous bodily harm.
The judge told the 50-year-old: “This was an entirely unnecessary as well as aggressive blow to a man who, at that moment, was incapable of defending himself because he was looking away.
“You know that serious injury can be caused by serious violence because you have a previous conviction for causing grievous bodily harm.
“It could easily have been worse. Everyone should be safe to attend matches like this without having to experience or witness violence.”
Lewis was previously convicted for assaulting a taxi driver in Cardiff in 2004, when he was visiting the city while it hosted that year’s FA Cup final between Millwall and Manchester United.
Defending Lewis, Matthew Sherratt QC said: “He has let himself down, he has let his family down and he has let his son down. He should never have punched Mr O’Donnell and he is sorry.”