UK light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight 10-round contest at the boxing arena.
But the challenger had his moments, connecting with powerful single shots in a fight where the styles never quite clicked, with constant grabbing marring the fight.
Two judges scored it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but what defines champions? They prevail," the victor commented in the ring as he was met by disapproval from the crowd.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the victory but the British boxer suffered the second defeat of his 28-fight professional journey.
The Champion Earns Victory in Slow Encounter
His career development has plateaued in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers prepared to take risks in a calculated early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a solid defense to barely take the better stanzas.
The spectators' acceptance decreased because of frequent holding, with the challenger slipping to the canvas various instances during clinches.
Parker finished the stronger, but multiple periods were hard to judge. Although, former world champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur beat Bradley Rea by divided decision in a outstanding local rivalry fight to secure the European light-heavyweight title.
The arena became quiet for a extremely emotional memorial prior to the bout as a bell ceremony rang in honor of local hero Hatton, who died 46 years old in recently.
It was the premier televised boxing event in the city since Hatton's death. As the concluding sound faded, the quiet gave way to a shout of "only one Hatton".
The fighting within the ring equaled the emotion. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the initial stage, rising shakily to his position on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was damaged soon, significant bruising appearing under his face as the fighters exchanged punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity rarely dropped over 12 rounds.
The victor narrowly won the result with scoring tallies of even score, close margin and wider gap, but the two athletes were persistent and courageous, everything their inspiration Hatton stood for.
