British light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a split decision in a close ten-round bout at the Co-op Live Arena.
But Parker had his moments, connecting with powerful individual punches in a fight where the approaches never quite clicked, with excessive holding marring the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They succeed," Buatsi commented in the arena as he was greeted with boos from the crowd.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts felt Parker, 31, deserved the victory but the British boxer experienced the second defeat of his 28-bout career.
Buatsi Secures Triumph in Uneventful Encounter
Buatsi's progress has plateaued in recent times through contract problems and considerable time of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what seems like a last chance - under recent support.
He started carefully, with neither fighter willing to be aggressive in a calculated opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who operated with a solid defense to narrowly win the cleaner stanzas.
The crowd's patience diminished during constant clinching, with Parker going down multiple occasions during clinches.
Parker finished the stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Fight
In the co-main event, Arthur beat Bradley Rea by close verdict in a outstanding local rivalry fight to win the European light-heavyweight belt.
The venue fell silent for a profoundly moving memorial ahead of the contest as a bell ceremony rang in honor of local hero Ricky Hatton, who died at age 46 in recently.
It was the premier televised match card in the urban center since Hatton's passing. As the concluding sound disappeared, the stillness gave way to a roar of "there's only one Ricky Hatton".
The action inside the ropes reflected the feeling. The eventual winner was floored by a left hook flush on the face in the second round, standing uncertainly to his position on the referee's count.
But the red-colored equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up quickly, noticeable damage forming under his eye as the fighters landed hard punches.
Rea, 27, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur edged the result with official scores of draw, 115-113 and clearer margin, but both fighters were determined and brave, everything their idol Hatton represented.
