UK light-heavyweight Joshua Buatsi made a winning yet unimpressive start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a tight ten-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a fight where the approaches didn't complement each other, with excessive holding disrupting the action.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi stated in the arena as he was faced negative reactions from the audience.
The British fighter got back to winning after his first professional defeat - 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 contender endured the second setback of his 28-bout career.
Buatsi Secures Win in Dull Encounter
The fighter's advancement has slowed in the past few years through management issues and long spells of lack of fights.
Previously predicted as the next star of British boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers ready to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second bout at light heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who utilized a tight guard to narrowly win the cleaner rounds.
The audience's tolerance decreased during constant clinching, with Parker slipping to the canvas several times during grappling.
The underdog ended stronger, but multiple periods were hard to judge. Although, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the secondary fight, Arthur beat Bradley Rea by split-decision in a terrific local rivalry battle to win the continental title.
The arena grew hushed for a deeply poignant memorial before the fight as a bell ceremony sounded in honor of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised fight night in the city since Hatton's passing. As the final chime ended, the stillness gave way to a chant of "there's only one Ricky Hatton".
The fighting within the ring equaled the feeling. The eventual winner was floored by a left hook squarely on the face in the early round, standing uncertainly to his position on the referee's count.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was damaged soon, significant bruising developing under his eye as the fighters traded hard punches.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
Arthur barely secured the verdict with official scores of draw, narrow lead and clearer margin, but both fighters were persistent and courageous, everything their idol Hatton stood for.
