British light-heavyweight Joshua Buatsi made a winning yet unimpressive start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round bout at the Manchester venue.
But Parker had his moments, delivering effective single shots in a fight where the approaches failed to mesh, with constant grabbing marring the contest.
The scoring judges had it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best display but how do champions respond? They win," the victor commented in the ringside as he was faced disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Several boxing experts thought the challenger, 31, deserved the victory but the British boxer suffered the second loss of his 28-bout professional journey.
Buatsi Secures Win in Slow Encounter
Buatsi's progress has stalled in recent years through contract problems and long spells of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to realize his potential - possibly in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors willing to overcommit in a calculated opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second fight at light heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for Buatsi, who worked behind a solid defense to narrowly win the cleaner periods.
The crowd's patience decreased because of regular grabbing, with Parker slipping to the canvas various instances during clinches.
The underdog finished the stronger, but many rounds were difficult to score. However, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Motivates All-Manchester Contest
In the secondary fight, Arthur overcame Rea by split-decision in a terrific local rivalry contest to secure the continental championship.
The arena grew hushed for a profoundly moving tribute prior to the bout as ten bells sounded in remembrance of Manchester's own Hatton, who died 46 years old in recently.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the last bell disappeared, the stillness transitioned to a roar of "there's just one Hatton".
The fighting inside the ropes equaled the sentiment. The eventual winner was floored by a left hook squarely on the face in the second round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was injured quickly, heavy swelling appearing under his orbital area as the pair traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor barely secured the verdict with official scores of even score, narrow lead and wider gap, but each competitor were relentless and courageous, all the qualities their idol Hatton embodied.
