The 24-year-old pleaded not guilty to wounding Faamanu Levi with intent to cause grievous bodily harm at Wattle Grove late at night on October 25, 2019.
In his evidence to the District Court jury, he denied being the stabber saying when he heard "knife knife" he ran away scared for his own safety.
But the jurors took just over two hours to return a guilty verdict on Thursday afternoon, prompting prosecutor Emma Curran to apply for immediate detention.
Judge Nanette Williams adjourned the application until Monday because of the late hour and because Friday's industrial strike by prison officers would mean he couldn't be taken into custody if she so ruled.
Fainu's barrister Margaret Cunneen SC opposed the application saying he had not breached any bail conditions over the last three years.
"This obviously will be appealed as a perverse verdict in view of the evidence," she said.
Noting it would be surprising if full-time custody was not imposed, the judge ordered Fainu to report to police daily, live with his parents and not leave the address without one of them, and not contact any witnesses.
Tony Quach testified to seeing Fainu plunge a steak knife into the back of Mr Levi in the car park brawl, referring to him looking angry and wearing a sling.
The jury was told the footballer had recently undergone shoulder surgery and had his arm in a sling that evening.
The stabbing occurred after an earlier fight on the dancefloor of the alcohol-free charity event organised by the Church of the Latter Day Saints.
Ms Cunneen had submitted that in the heat of the moment, with the darkness, chaos and speed of it unfolding, the jury could not be certain who stabbed Mr Levi.
But they could be certain the NRL hooker, never before in trouble for any violence, did not, she said.
The jurors were told Fainu and his friend Uona Faingaa known as "Big Buck," were involved in the dancefloor fight and ushered outside by Mr Levi who told them not to fight on church grounds.
CCTV footage showed Fainu with a white towel draped over his head and his four friends jumping back over a fence and into a car park where the brawl occurred with Mr Levi.
Ms Curran contended that when things looked like they were getting out of hand, Fainu pulled out the knife and plunged it into Mr Levi's back.
"Not content with having stabbed him once ... (Fainu) moved around to the front of Mr Levi and swung the knife upwards towards (his) face cutting him across the eyebrow.
"Then he ran back to the car."
Another witness involved in the brawl told the court that he saw a male in a sling approach his brother holding a knife.
"The accused was the only person in a sling. And a sling is quite a distinctive feature," Ms Curran said.
She argued that Fainu was an unimpressive and evasive witness who gave contradictory and occasionally fanciful answers.
She asked the jury to reject such evidence including his reason for scaling a three-metre tall brick fence to get back onto the church grounds, rather than entering through the front gate, because "it was easier".
He told the jury the white towel draped over his head was a "security blanket" doused in cold water because he had a headache, not to hide his identity.
Fainu also denied holding, seeing, or knowing of a weapon being brought that evening.
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