UK light heavyweight Joshua Buatsi earned a win yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close 10-round bout at the boxing arena.
But the challenger found success, landing some telling single shots in a fight where the techniques failed to mesh, with excessive holding marring the action.
Two judges judged it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest display but what do champions do? They succeed," the victor stated in the ring as he was met by boos from the crowd.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts felt Parker, 31, deserved the decision but the contender experienced the second defeat of his 28-bout professional journey.
Buatsi Claims Victory in Uneventful Affair
His career development has plateaued in recent times through management issues and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what appears to be a last chance - under recent support.
He began cautiously, with both competitors ready to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had kept it close, making life awkward for Buatsi, who operated with a tight guard to barely take the better rounds.
The spectators' acceptance diminished amid regular grabbing, with Parker falling to the floor multiple occasions during clinches.
Parker finished the stronger, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor advances, but with the caliber of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Homage Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a terrific all-Manchester fight to secure the European light heavyweight title.
The stadium fell silent for a profoundly moving memorial prior to the bout as a bell ceremony rang in honor of Manchester's own Hatton, who died at age 46 in last month.
It was the premier televised boxing event in the location since Hatton's death. As the concluding sound ended, the silence became a roar of "there's only one Hatton".
The competition inside the ropes reflected the emotion. The eventual winner was floored by a significant blow flush on the face in the initial stage, standing uncertainly to his position on the eight count.
But the red-colored equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was damaged early, noticeable damage forming under his face as the duo traded hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, responded in kind with powerful upward punches as the pace hardly diminished over 12 rounds.
The victor narrowly won the decision with official scores of 114-114, close margin and wider gap, but both fighters were persistent and courageous, everything their hero Hatton embodied.
