The Australia midfielder looked close to tears and explained the goal had been dedicated to his brother Sean, jailed for six years in January after an attack that left his victim Chris Stapley partially blinded, says the Daily Mail.
The incident took place outside a cab office in Bromley in 2004.
After fleeing to Sydney while on bail, Cahill's brother was extradited and found guilty of grievous bodily harm.
The court heard he kicked Mr Stapley in the head twice while he lay on the floor.
Cahill, 28, said: "That was for my older brother because I am just proud that he is happy and I am happy. I just wanted to send a message to him. It has been emotional for me because my family mean so much to me."
