British light-heavyweight Joshua Buatsi made a winning yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a tight ten-round contest at the Co-op Live Arena.
But the challenger had his moments, landing some telling clean blows in a fight where the approaches never quite clicked, with excessive holding spoiling the fight.
Two judges had it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my finest showing but how do champions respond? They prevail," the victor stated in the ringside as he was greeted with boos from the audience.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th career victory.
Various analysts thought the challenger, 31, should have been awarded the victory but the Derbyshire fighter endured the second setback of his 28-match career.
Buatsi Secures Triumph in Uneventful Encounter
Buatsi's progress has slowed in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now finds himself trying to realize his potential - maybe in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers willing to overcommit in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand during the bout, was competing in only his second fight at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who worked behind a protective stance to edge the more effective periods.
The crowd's patience decreased because of frequent holding, with the challenger slipping to the canvas various instances during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. However, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
The victor moves on, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Inspires All-Manchester Contest
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a outstanding all-Manchester contest to secure the European light-heavyweight belt.
The arena grew hushed for a deeply poignant tribute before the fight as a bell ceremony rang in honor of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised match card in the location since the passing of Hatton. As the final chime faded, the quiet became a shout of "there's only one Hatton".
The action during the bout equaled the emotion. Arthur was knocked down by a left hook squarely on the face in the initial stage, getting up unsteadily to his footing on the count of eight.
But the traditional red attire of the fighter soon advanced again. From the other corner, the opponent - in competitive blue - was damaged soon, noticeable damage forming under his face as the pair landed hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over 12 rounds.
The victor edged the verdict with judges scorecards of even score, 115-113 and wider gap, but each competitor were relentless and full of heart, all the qualities their hero the legend embodied.
