The fast-food outlet in Leicester city centre was on lockdown following the attack shortly after 4.30am last Tuesday.
A video shot in the restaurant shortly before the violence erupted is now being investigated by Leicestershire Police after it was viewed 55,000 times on social media.
The nine-second clip appears to show a man in a black puffer jacket holding a knife in his right hand as he is confronted by three men inside the restaurant.
The three black youths also appear to be armed with weapons, including one who waved his leather belt at the knifeman.
A video was shot in the restaurant shortly before the violence erupted
The mass fight involving 12 people resulted in a 21-year-old man suffering a stab wound. He was rushed to hospital but was later released.
Bloodied napkins and rubbish was seen strewn across the floor of the restaurant after police sealed it off as a crime scene.
The mass fight involved 12 people
A police spokesman said: "A video that has been circulating on social media has been brought to our attention.
"It has been passed on to officers who will be making enquiries to ascertain if it is linked to any incident."