UK light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a fight where the styles failed to mesh, with excessive holding spoiling the fight.
The scoring judges scored it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my finest performance but what defines champions? They win," the victor commented in the ringside as he was faced negative reactions from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers felt Parker, 31, should have been awarded the win but the British boxer suffered the second defeat of his 28-bout career.
Buatsi Earns Win in Uneventful Encounter
Buatsi's progress has plateaued in the past few years through management issues and considerable time of lack of fights.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers prepared to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who worked behind a tight guard to barely take the cleaner stanzas.
The spectators' acceptance decreased during frequent holding, with Parker going down various instances during clinches.
Parker finished the stronger, but multiple periods were hard to judge. However, former world champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Hatton Memorial Fuels All-Manchester Contest
In the supporting bout, Arthur beat Rea by split-decision in a excellent local rivalry contest to secure the European light heavyweight belt.
The arena fell silent for a deeply poignant memorial before the fight as a bell ceremony rang in memory of Manchester's own Hatton, who died at age 46 in last month.
It was the first major televised boxing event in the city since Hatton's death. As the last bell faded, the stillness gave way to a shout of "there's only one Hatton".
The action inside the ropes reflected the feeling. The eventual winner was knocked down by a left hook directly to the face in the second round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his face as the pair traded hard punches.
The challenger, twenty-seven, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the intensity seldom decreased over 12 rounds.
Arthur narrowly won the result with official scores of 114-114, 115-113 and 115-112, but the two athletes were determined and full of heart, everything their hero the legend embodied.
