British light-heavyweight Joshua Buatsi secured a victory yet disappointing start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a close ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a contest where the techniques never quite clicked, with frequent clinching marring the action.
The officials judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest display but what defines champions? They win," the victor commented in the ringside as he was met by negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers felt the challenger, 31, merited the decision but the British boxer experienced the second loss of his 28-match career.
The Champion Claims Victory in Uneventful Match
His career development has slowed in recent times through management issues and extended periods of inactivity.
Previously predicted as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors prepared to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who operated with a tight guard to barely take the more effective periods.
The crowd's patience decreased amid regular grabbing, with Parker slipping to the canvas several times during clinches.
The underdog ended stronger, but several stanzas were difficult to score. However, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor advances, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Hatton Homage Inspires Local Rivalry Battle
In the secondary fight, Arthur overcame Rea by close verdict in a excellent Manchester vs Manchester battle to win the European light-heavyweight title.
The arena grew hushed for a profoundly moving memorial ahead of the contest as a bell ceremony sounded in honor of Manchester's own Hatton, who passed away at age 46 in last month.
It was the first major televised fight night in the location since the passing of Hatton. As the final chime faded, the silence transitioned to a roar of "there's just one Hatton".
The action within the ring equaled the feeling. Arthur was knocked down by a powerful punch directly to the chin in the early round, rising shakily to his position on the eight count.
But the red-colored shorts of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was damaged quickly, heavy swelling appearing under his face as the fighters traded hard punches.
Rea, 27, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the action rarely dropped over twelve rounds.
Arthur edged the result with scoring tallies of draw, 115-113 and wider gap, but the two athletes were relentless and brave, everything their inspiration the legend embodied.
