The stab victim at the West Ham v Millwall match was an innocent family man supporting Millwall who was attacked by rival fans, police said.
The 43-year-old remains in a stable condition in hospital after being stabbed in the chest during the disorder surrounding Tuesday's Carling Cup tie, Scotland Yard said.
He was attacked after arriving at Upton Park with his family to watch the game, a Yard spokesman said.
"There were 10 members of the family in total, including the victim's sons, aged 17 and 18.
"The family are Millwall supporters. They initially headed to the wrong entrance of the ground and were making their way back to the correct entrance when they were approached by a large group of West Ham supporters who pursued them along Tudor Road.
"The victim and his sons were separated from the rest of the family and attacked by the group who threw punches and kicks.
"The victim was further separated from his sons and continued to be attacked by the group, receiving a single stab wound to the chest.
"The group fled in various directions. The victim managed to make his way through the alleyway at the end of Tudor Road and collapsed by the Priory Road bus garage.
"He was found by his sons who alerted the emergency services. The victim's sons were not seriously injured and did not require hospital treatment."
Investigating officer Det Insp Lee Barnard of Newham Violent Crime Unit said: "This incident was not a pre-arranged act of violence between two sets of rival supporters; this was an innocent family man who was subjected to a senseless attack by people intent on causing violence.
"Although the family were not wearing any clothing that would identify them as Millwall supporters, their direction of travel may have indicated that they were.
"The victim was subjected to a brutal and sustained attack as he and his family tried to flee to safety. If not for the swift intervention of paramedics, the man would have lost his life.
"I urge anyone who witnessed this attack, or has information that can assist this investigation, to contact us."
Anyone with information was asked to call the Violent Crime Unit at Plaistow Police Station on 020 7275 5851, or, to remain anonymous, Crimestoppers on 0800 555 111.
The mass violence surrounding the match has been condemned by police, politicians and the clubs involved.
Thirteen people were arrested after clashes between rival supporters inside and outside the ground.
The Football Association has asked for life bans for those identified as being involved.
Violence broke out shortly after 6pm as thousands of fans made their way to the ground.
Onlookers described bricks and bottles being lobbed as hundreds of supporters rioted.
Inside the ground fans fought with police and stewards and play was suspended when supporters invaded the pitch.