UK light heavyweight Joshua Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a split decision in a close 10-round bout at the boxing arena.
But Parker landed significant shots, delivering effective single shots in a match where the styles failed to mesh, with frequent clinching disrupting the contest.
The officials judged it 96-94 to the winner, with the final judge a even score.
"It wasn't my finest showing but how do champions respond? They win," the victor said in the ring as he was greeted with boos from the crowd.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers thought the challenger, 31, should have been awarded the victory but the British boxer experienced the second loss of his 28-match professional journey.
The Champion Claims Win in Slow Encounter
The fighter's advancement has stalled in recent years through contract problems and long spells of lack of fights.
Once tipped as the rising talent of British boxing, he now finds himself trying to achieve expectations - possibly in what seems like a final roll of the dice - under recent support.
He began cautiously, with the two boxers prepared to take risks in a calculated early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second fight at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who utilized a protective stance to edge the cleaner periods.
The crowd's patience wore thin amid constant clinching, with the challenger falling to the floor multiple occasions during grappling.
The underdog ended stronger, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Homage Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a excellent local rivalry battle to secure the European light heavyweight belt.
The arena grew hushed for a profoundly moving homage ahead of the contest as 10 bells tolled in remembrance of local hero Hatton, who succumbed at age 46 in last month.
It was the premier televised boxing event in the city since Hatton's death. As the last bell faded, the quiet transitioned to a shout of "only one Hatton".
The fighting during the bout equaled the sentiment. The eventual winner was floored by a left hook flush on the chin in the early round, rising shakily to his feet on the count of eight.
But the red-colored attire of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was injured early, heavy swelling developing under his face as the pair traded punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over twelve rounds.
Arthur barely secured the result with official scores of draw, close margin and clearer margin, but each competitor were persistent and courageous, everything their idol Hatton embodied.
