The victim was taken to Wanganui Hospital with multiple fractures to his eye socket and a head wound requiring stitches.
Both Sinclair and his mother, sitting in the public gallery, began to cry as details of the attack were read out to the court.
It is the second time Sinclair has been sent to prison for such an offence. In 2009 he was sentenced to two years in jail for injuring with intent to cause grievous bodily harm.
His lawyer, Stephanie Burlace, said he had tried to make serious changes in his life since his release from prison, including moving out of the area and cutting out alcohol almost completely. This was the first time in a long time he had drunk too much and gone out, she said.
He accepted alcohol was an issue and was upset and remorseful over what had happened and worried about losing his job.
Judge Matheson said Sinclair was a dedicated and respected member of a martial arts club, which made the offending worse given the ethical standards he would have been taught there.
His employer had written to the court to say he was considered hard-working and trustworthy, but his pre-sentence report deemed him a high risk of reoffending and the attack was too serious to warrant a sentence of home detention.
Sinclair must attend anger management and drug and alcohol counselling, undergo a psychological assessment and undertake any other programmes as directed by probation.