British light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a competitive 10-round contest at the boxing arena.
But the challenger found success, connecting with powerful individual punches in a fight where the techniques never quite clicked, with constant grabbing marring the contest.
Two judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but what defines champions? They succeed," the victor said in the ring as he was met by boos from the audience.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Various analysts thought the challenger, thirty-one, should have been awarded the victory but the British boxer suffered the second defeat of his 28-match career.
The Champion Secures Triumph in Slow Encounter
Buatsi's progress has slowed in recent years through contract problems and long spells of inactivity.
Previously predicted as the rising talent of UK boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors willing to be aggressive in a tentative opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second appearance at light heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for Buatsi, who utilized a solid defense to barely take the cleaner rounds.
The spectators' acceptance wore thin amid frequent holding, with the challenger falling to the floor several times during clinches.
The underdog finished the stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Inspires Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a outstanding local rivalry battle to win the European light heavyweight championship.
The arena grew hushed for a profoundly moving memorial ahead of the contest as 10 bells rang in memory of the city's native Ricky Hatton, who died at age 46 in September.
It was the premier televised fight night in the city since the passing of Hatton. As the last bell ended, the quiet transitioned to a shout of "only one Hatton".
The action within the ring equaled the emotion. The eventual winner was floored by a powerful punch directly to the face in the initial stage, rising shakily to his footing on the referee's count.
But the traditional red attire of Arthur soon advanced again. On the opposite side, the opponent - in rival City blue - was damaged quickly, noticeable damage forming under his orbital area as the pair traded hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity hardly diminished over the full distance.
Arthur barely secured the decision with scoring tallies of draw, close margin and wider gap, but both fighters were determined and courageous, everything their idol Hatton represented.
