British light heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a competitive 10-round contest at the Manchester venue.
But the challenger found success, connecting with powerful clean blows in a contest where the techniques didn't complement each other, with constant grabbing spoiling the action.
The officials had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but how do champions respond? They prevail," the victor said in the ringside as he was greeted with disapproval from the spectators.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers thought the challenger, thirty-one, should have been awarded the victory but the Derbyshire fighter endured the second defeat of his 28-bout career.
Buatsi Earns Victory in Dull Match
The fighter's advancement has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what feels like a last chance - under fresh management.
He started carefully, with the two boxers prepared to overcommit in a tentative early stages.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to barely take the cleaner periods.
The crowd's patience wore thin because of regular grabbing, with the challenger going down various instances during grappling.
Parker closed well, but many rounds were hard to judge. However, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Homage Inspires Local Rivalry Battle
In the supporting bout, Arthur overcame Rea by split-decision in a outstanding local rivalry contest to capture the European light heavyweight championship.
The venue became quiet for a profoundly moving homage ahead of the contest as a bell ceremony sounded in honor of the city's native Ricky Hatton, who died at age 46 in recently.
It was the first major televised fight night in the urban center since Hatton's death. As the concluding sound faded, the quiet became a roar of "there's only one Ricky Hatton".
The fighting within the ring equaled the sentiment. Arthur was sent to the canvas by a left hook squarely on the face in the initial stage, standing uncertainly to his position on the count of eight.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged quickly, significant bruising developing under his face as the pair exchanged punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the action seldom decreased over 12 rounds.
The victor barely secured the decision with scoring tallies of draw, close margin and 115-112, but the two athletes were determined and full of heart, everything their hero Hatton embodied.
