UK light heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a tight 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, landing some telling individual punches in a contest where the approaches never quite clicked, with constant grabbing disrupting the action.
The scoring judges had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest performance but what do champions do? They prevail," Buatsi said in the arena as he was met by boos from the crowd.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers felt the challenger, thirty-one, merited the decision but the British boxer suffered the second defeat of his 28-fight professional journey.
The Champion Secures Triumph in Uneventful Match
Buatsi's progress has stalled in the past few years through contract problems and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with both competitors willing to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at light-heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a tight guard to narrowly win the better stanzas.
The crowd's patience wore thin during regular grabbing, with the challenger slipping to the canvas various instances during close exchanges.
The underdog ended stronger, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, giving Buatsi just a single round.
The victor moves on, but with the presence of heavy hitter Benavidez, and Russian champion Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Hatton Memorial Motivates Manchester Derby Contest
In the secondary fight, Arthur beat Bradley Rea by split-decision in a excellent Manchester vs Manchester fight to capture the continental championship.
The arena fell silent for a extremely emotional tribute ahead of the contest as a bell ceremony rang in honor of the city's native Hatton, who died aged 46 in September.
It was the first major televised fight night in the location since Hatton's death. As the final chime faded, the silence became a roar of "there's only one Ricky Hatton".
The action during the bout matched the emotion. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the initial stage, getting up unsteadily to his footing on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was injured quickly, heavy swelling appearing under his eye as the fighters traded powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor narrowly won the verdict with official scores of draw, narrow lead and clearer margin, but both fighters were determined and full of heart, everything their idol Hatton represented.
