UK light heavyweight Joshua Buatsi earned a win yet disappointing start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a competitive ten-round fight at the Co-op Live Arena.
But the challenger had his moments, delivering effective single shots in a fight where the styles failed to mesh, with excessive holding spoiling the contest.
The scoring judges scored it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but how do champions respond? They succeed," the victor stated in the ringside as he was greeted with negative reactions from the spectators.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Various analysts felt the challenger, thirty-one, merited the victory but the British boxer experienced the second loss of his 28-match career.
The Champion Secures Triumph in Slow Affair
The fighter's advancement has slowed in the past few years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the next star of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors willing to be aggressive in a tentative opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who utilized a protective stance to barely take the more effective stanzas.
The audience's tolerance decreased during regular grabbing, with Parker going down multiple occasions during clinches.
The underdog finished the stronger, but several stanzas were difficult to score. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Memorial Inspires Local Rivalry Battle
In the supporting bout, Arthur defeated Rea by divided decision in a excellent all-Manchester battle to secure the European light heavyweight belt.
The stadium grew hushed for a profoundly moving memorial ahead of the contest as 10 bells sounded in honor of the city's native Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast match card in the city since Hatton's death. As the last bell faded, the stillness gave way to a roar of "there's just one Ricky Hatton".
The competition within the ring matched the sentiment. Arthur was sent to the canvas by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his footing on the count of eight.
But the traditional red shorts of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was damaged early, noticeable damage forming under his eye as the pair traded punishing blows.
The challenger, 27, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the result with scoring tallies of 114-114, 115-113 and clearer margin, but both fighters were relentless and courageous, exactly what their hero Hatton embodied.
