Jimmy was killed a day after his 16th birthday
The teenager accused of killing 16-year-old Jimmy Mizen was in a state of panic after the confrontation, a court has heard.
Jake Fahri, 19, from Lee, south-east London, is accused of killing Jimmy at a bakery in Lee on 10 May last year.
Mr Fahri's friend Daniel Heaven told the Old Bailey Mr Fahri was pale and panicking after the incident.
Mr Heaven told the court: "He said 'I think I cut him. I seen a lot of blood.'" Mr Fahri denies murder.
Jimmy bled to death after being attacked at the Three Cooks bakery store with his brother Tommy.
Musician Mr Heaven told the court that Mr Fahri telephoned him later that day.
Mr Heaven went to see Mr Fahri, who told him he had got involved in a fight with two youths after going to the shop to buy a sandwich.
Mr Fahri told him that he had picked up a sign but Jimmy tried to take it away from him, Mr Heaven said.
"He looked for something else and hit him with it," Mr Heaven told the court.
"It broke and cut the other boy's neck."
Mr Fahri did not say what item he picked up, Mr Heaven added.
The court has heard that Jimmy died after an oven-proof dish shattered and cut vital blood vessels in his neck.
The trial continues.