British light heavyweight Joshua Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close 10-round fight at the boxing arena.
But Parker found success, delivering effective individual punches in a match where the styles never quite clicked, with excessive holding spoiling the contest.
The scoring judges scored it 96-94 to the winner, with the third a even score.
"It wasn't my greatest display but how do champions respond? They prevail," the victor said in the ring as he was greeted with disapproval from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts felt Parker, thirty-one, deserved the decision but the contender endured the second setback of his 28-bout professional journey.
The Champion Earns Victory in Dull Encounter
His career development has plateaued in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers prepared to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for Buatsi, who utilized a solid defense to edge the better rounds.
The audience's tolerance diminished during constant clinching, with the challenger going down several times during clinches.
The underdog closed well, but multiple periods were difficult to score. However, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Tribute Inspires All-Manchester Battle
In the co-main event, Arthur defeated Bradley Rea by split-decision in a outstanding local rivalry contest to win the European light-heavyweight belt.
The venue grew hushed for a deeply poignant tribute prior to the bout as a bell ceremony rang in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the premier televised fight night in the city since Hatton's death. As the last bell ended, the silence became a roar of "there's only one Hatton".
The action inside the ropes reflected the sentiment. Arthur was knocked down by a powerful punch squarely on the chin in the early round, standing uncertainly to his footing on the referee's count.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged quickly, significant bruising forming under his face as the pair landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
Arthur edged the result with judges scorecards of draw, 115-113 and 115-112, but both fighters were determined and brave, everything their hero Hatton represented.
